Actualizar datos de un datagrid en visual basic 2005 y mysql

Hola amigos de yoreparo, espero me puedan ayudar con la siguiente duda:
Estoy haciendo un sistema de control de inventario de un videoclub en visual basic 2005 y mysql, mis registros los guarda a la perfección pero si yo quiero modificar esos datos y doy clic al botón guardar modificación no me permite hacerlo, allí dejé mi código de guardar modificación en comentarios, espero me puedan ayudar a formular el código correcto, gracias.

Imports MySql.Data.MySqlClient
Imports System.Data

Public Class rentadepeliculas
Dim con As New MySqlConnection
Dim ds As New DataSet
Private Sub borrarcajas()
tnocredencial.Text = ""
tfolio.Text = ""
dfecha.Value = Date.Now
tnombre.Text = ""
tdireccion.Text = ""
ttelefono.Text = ""
DataGridView1.Rows.Clear()
DataGridView1.Rows.Add(10)
tcantidadconletra.Text = ""
tsubtotal.Text = ""
tiva.Text = ""
ttotal.Text = ""
End Sub
Private Sub habilitarcajas()
tnocredencial.Enabled = True
dfecha.Enabled = True
DataGridView1.Enabled = True
End Sub
Private Sub deshabilitarcajas()
tnocredencial.Enabled = False
dfecha.Enabled = True
DataGridView1.Enabled = False
End Sub

Private Sub rentadepeliculas_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
con = New MySqlConnection("server=X;database=bdX;uid=X;pwd=XXXXX")
con.Open()
DataGridView1.Rows.Add(10)
End Sub

Private Sub lguardar_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles lguardar.Click
Dim cnrm As New MySqlDataAdapter
Dim cnrd As New MySqlDataAdapter
Dim dsrentas As New DataSet
Dim filarentas As DataRow
Dim sincrentas As New MySqlCommandBuilder
Dim comrentas As New MySqlCommand
cnrm = New MySqlDataAdapter("select * from rentadepeliculas_maestro order by Folio;", con)
cnrm.Fill(dsrentas, "rentadepeliculas_maestro")
sincrentas = New MySqlCommandBuilder(cnrm)
If tnocredencial.Text = "" Then
MsgBox("Es necesario que ingrese el número de credencial.", MsgBoxStyle.Information, "Operación cancelada.")
Exit Sub
End If
For i As Integer = 0 To DataGridView1.Rows.Count - 1
If DataGridView1.Rows(i).Cells(0).Value = "" And DataGridView1.Rows(i).Cells(4).Value = "" And DataGridView1.Rows(i).Cells(5).Value = "" And tcantidadconletra.Text = "" And tsubtotal.Text = "" And tiva.Text = "" And ttotal.Text = "" Then
MsgBox("No puede realizar un registro de renta con campos en blanco.", MsgBoxStyle.Information, "Operación cancelada.")
Exit Sub
End If
Next
If dsrentas.Tables("rentadepeliculas_maestro").Rows.Count > 0 Then
filarentas = dsrentas.Tables("rentadepeliculas_maestro").Rows(dsrentas.Tables("rentadepeliculas_maestro").Rows.Count - 1)
tfolio.Text = Val(filarentas("Folio")) + 1
Else
tfolio.Text = "1"
End If
filarentas = dsrentas.Tables("rentadepeliculas_maestro").NewRow
filarentas("No_credencial") = tnocredencial.Text
filarentas("Folio") = tfolio.Text
filarentas("Fecha") = dfecha.Text
filarentas("Nombre") = tnombre.Text
filarentas("Direccion") = tdireccion.Text
filarentas("Telefono") = ttelefono.Text
filarentas("Cantidad_letra") = tcantidadconletra.Text
filarentas("Subtotal") = Val(tsubtotal.Text)
filarentas("Iva") = Val(tiva.Text)
filarentas("Total") = Val(ttotal.Text)
dsrentas.Tables("rentadepeliculas_maestro").Rows.Add(filarentas)
cnrm.Update(dsrentas.Tables("rentadepeliculas_maestro"))
dsrentas.AcceptChanges()
cnrd = New MySqlDataAdapter("select * from rentadepeliculas_detalle order by Num;", con)
cnrd.Fill(dsrentas, "rentadepeliculas_detalle")
sincrentas = New MySqlCommandBuilder(cnrd)
For i As Integer = 0 To DataGridView1.Rows.Count - 1
If DataGridView1.Rows(i).Cells(1).Value <> "" Then
filarentas = dsrentas.Tables("rentadepeliculas_detalle").NewRow
filarentas("Folio") = tfolio.Text
filarentas("Ean") = DataGridView1.Rows(i).Cells(0).Value
filarentas("Titulo") = DataGridView1.Rows(i).Cells(1).Value
filarentas("Formato") = DataGridView1.Rows(i).Cells(2).Value
filarentas("Contenido") = CDbl(DataGridView1.Rows(i).Cells(3).Value)
filarentas("Precio") = CDbl(DataGridView1.Rows(i).Cells(4).Value)
filarentas("Copias") = CDbl(DataGridView1.Rows(i).Cells(5).Value)
filarentas("Importe") = CDbl(DataGridView1.Rows(i).Cells(6).Value)
dsrentas.Tables("rentadepeliculas_detalle").Rows.Add(filarentas)
cnrd.Update(dsrentas.Tables("rentadepeliculas_detalle"))
dsrentas.AcceptChanges()
End If
Next
deshabilitarcajas()
lnuevo.Enabled = True
lguardar.Enabled = False
limprimir.Enabled = True
lbuscar.Enabled = True
lmodificar.Enabled = True
lguardarmodificacion.Enabled = False
leliminar.Enabled = False
lcancelar.Enabled = False
End Sub

Private Sub lbuscar_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles lbuscar.Click
Dim cnrentas As New MySqlDataAdapter
Dim dsrent As New DataSet
Dim filarent As DataRow
Dim x As String
x = InputBox("Introduzca el número de folio que desea buscar.")
If x = "" Then Exit Sub
cnrentas = New MySqlDataAdapter("select * from rentadepeliculas_maestro where Folio = " & x & ";", con)
cnrentas.Fill(dsrent, "rentadepeliculas_maestro")
If dsrent.Tables("rentadepeliculas_maestro").Rows.Count > 0 Then
filarent = dsrent.Tables("rentadepeliculas_maestro").Rows(0)
tnocredencial.Text = filarent("No_credencial")
tfolio.Text = filarent("Folio")
dfecha.Text = filarent("Fecha")
tnombre.Text = filarent("Nombre")
tdireccion.Text = filarent("Direccion")
ttelefono.Text = filarent("Telefono")
tcantidadconletra.Text = filarent("Cantidad_letra")
tsubtotal.Text = filarent("Subtotal")
tiva.Text = filarent("Iva")
ttotal.Text = filarent("Total")
Else
MsgBox("Folio no encontrado.", MsgBoxStyle.Information, "Operación cancelada.")
borrarcajas()
lnuevo.Enabled = True
lguardar.Enabled = False
limprimir.Enabled = False
lbuscar.Enabled = True
lmodificar.Enabled = False
lguardarmodificacion.Enabled = False
leliminar.Enabled = False
lcancelar.Enabled = False
Exit Sub
dsrent.Clear()
dsrent.Dispose()
cnrentas.Dispose()
End If
DataGridView1.Rows.Clear()
DataGridView1.Rows.Add(10)
cnrentas = New MySqlDataAdapter("select * from rentadepeliculas_detalle where Folio =" & x & ";", con)
cnrentas.Fill(dsrent, "rentadepeliculas_detalle")
If dsrent.Tables("rentadepeliculas_detalle").Rows.Count > 0 Then
For i As Integer = 0 To dsrent.Tables("rentadepeliculas_detalle").Rows.Count - 1
filarent = dsrent.Tables("rentadepeliculas_detalle").Rows(i)
DataGridView1.Rows(i).Cells(0).Value = filarent("Ean")
DataGridView1.Rows(i).Cells(1).Value = filarent("Titulo")
DataGridView1.Rows(i).Cells(2).Value = filarent("Formato")
DataGridView1.Rows(i).Cells(3).Value = filarent("Contenido")
DataGridView1.Rows(i).Cells(4).Value = filarent("Precio")
DataGridView1.Rows(i).Cells(5).Value = filarent("Copias")
DataGridView1.Rows(i).Cells(6).Value = filarent("Importe")
lnuevo.Enabled = False
lguardar.Enabled = False
limprimir.Enabled = True
lbuscar.Enabled = True
lmodificar.Enabled = True
lguardarmodificacion.Enabled = False
leliminar.Enabled = True
lcancelar.Enabled = True
Next
End If
dsrent.Clear()
dsrent.Dispose()
cnrentas.Dispose()
End Sub

Private Sub lmodificar_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles lmodificar.Click
habilitarcajas()
tnocredencial.Focus()
lnuevo.Enabled = False
lguardar.Enabled = False
limprimir.Enabled = False
lbuscar.Enabled = False
lmodificar.Enabled = False
lguardarmodificacion.Enabled = True
leliminar.Enabled = False
lcancelar.Enabled = True
End Sub

Private Sub lguardarmodificacion_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles lguardarmodificacion.Click
'Dim cnrm As New MySqlDataAdapter
'Dim cnrd As New MySqlDataAdapter
'Dim dsrentas As New DataSet
'Dim filarentas As DataRow
'Dim sincrentas As New MySqlCommandBuilder
'Dim comrentas As New MySqlCommand
'cnrm = New MySqlDataAdapter("select * from rentadepeliculas_maestro order by Folio;", con)
'cnrm.Fill(dsrentas, "rentadepeliculas_maestro")
'sincrentas = New MySqlCommandBuilder(cnrm)
'filarentas = dsrentas.Tables("rentadepeliculas_maestro").NewRow
'filarentas.BeginEdit()
'filarentas("No_credencial") = tnocredencial.Text
'filarentas("Folio") = tfolio.Text
'filarentas("Fecha") = dfecha.Text
'filarentas("Nombre") = tnombre.Text
'filarentas("Direccion") = tdireccion.Text
'filarentas("Telefono") = ttelefono.Text
'filarentas("Cantidad_letra") = tcantidadconletra.Text
'filarentas("Subtotal") = Val(tsubtotal.Text)
'filarentas("Iva") = Val(tiva.Text)
'filarentas("Total") = Val(ttotal.Text)
'filarentas.EndEdit()
'cnrm.Update(dsrentas.Tables("rentadepeliculas_maestro"))
'dsrentas.AcceptChanges()
'cnrd = New MySqlDataAdapter("select * from rentadepeliculas_detalle order by Num;", con)
'cnrd.Fill(dsrentas, "rentadepeliculas_detalle")
'sincrentas = New MySqlCommandBuilder(cnrd)
'For i As Integer = 0 To DataGridView1.Rows.Count - 1
' If DataGridView1.Rows(i).Cells(1).Value <> "" Then
' filarentas = dsrentas.Tables("rentadepeliculas_detalle").NewRow
' filarentas.BeginEdit()
' filarentas("Folio") = tfolio.Text
' filarentas("Ean") = DataGridView1.Rows(i).Cells(0).Value
' filarentas("Titulo") = DataGridView1.Rows(i).Cells(1).Value
' filarentas("Formato") = DataGridView1.Rows(i).Cells(2).Value
' filarentas("Contenido") = CDbl(DataGridView1.Rows(i).Cells(3).Value)
' filarentas("Precio") = CDbl(DataGridView1.Rows(i).Cells(4).Value)
' filarentas("Copias") = CDbl(DataGridView1.Rows(i).Cells(5).Value)
' filarentas("Importe") = CDbl(DataGridView1.Rows(i).Cells(6).Value)
' filarentas.EndEdit()
' cnrm.Update(dsrentas.Tables("rentadepeliculas_detalle"))
' dsrentas.AcceptChanges()
' End If
'Next
'deshabilitarcajas()
'lnuevo.Enabled = True
'lguardar.Enabled = False
'limprimir.Enabled = True
'lbuscar.Enabled = True
'lmodificar.Enabled = True
'lguardarmodificacion.Enabled = False
'leliminar.Enabled = False
'lcancelar.Enabled = False
End Sub

Private Sub tnocredencial_KeyPress(ByVal sender As Object, ByVal e As System.Windows.Forms.KeyPressEventArgs) Handles tnocredencial.KeyPress
Dim cncliente As New MySqlDataAdapter
Dim dscliente As New DataSet
Dim filacliente As DataRow
''If Asc(e.KeyChar) = Keys.Return Then
If Asc(e.KeyChar) = 13 Then
cncliente = New MySqlDataAdapter("select Nombres, Apellido_paterno, Apellido_materno, Colonia, Calle, Ciudad, Telefono from registrodeclientes where No_credencial='" & tnocredencial.Text & "';", con)
cncliente.Fill(dscliente, "registrodeclientes")
If dscliente.Tables("registrodeclientes").Rows.Count > 0 Then
filacliente = dscliente.Tables("registrodeclientes").Rows(0)
tnombre.Text = filacliente("Nombres") & " " & filacliente("Apellido_paterno") & " " & filacliente("Apellido_materno")
tdireccion.Text = filacliente("Colonia") & " " & filacliente("Calle") & " " & filacliente("Ciudad")
ttelefono.Text = filacliente("Telefono")
Else
MsgBox("Cliente no encontrado.", MsgBoxStyle.Information, "Búsqueda de clientes.")
tnocredencial.Text = ""
tnombre.Text = ""
tdireccion.Text = ""
ttelefono.Text = ""
End If
dscliente.Clear()
dscliente.Dispose()
cncliente.Dispose()
'dscliente.Tables("registrodeclientes").Clear()
End If
End Sub

Private Sub DataGridView1_CellEndEdit(ByVal sender As Object, ByVal e As System.Windows.Forms.DataGridViewCellEventArgs) Handles DataGridView1.CellEndEdit
Dim cnpel As New MySqlDataAdapter
Dim dspel As New DataSet
Dim filapel As DataRow
Dim ean As String
Dim precio As Double
Dim copias As Double
Dim vsubt As Double = 0
Dim vean As String = ""
Dim vlinea As Integer
Select Case e.ColumnIndex
Case 0
vean = DataGridView1.CurrentCell.Value
vlinea = DataGridView1.CurrentRow.Index
For i As Integer = 0 To DataGridView1.Rows.Count - 1
If vean = DataGridView1.Rows(i).Cells(0).Value And i <> vlinea Then
MsgBox("No puede introducir un mismo ean.", MsgBoxStyle.Information, "Operación cancelada.")
DataGridView1.CurrentCell.Value = ""
SendKeys.Send("{up}")
Exit Sub
End If
Next
ean = DataGridView1.Rows(DataGridView1.CurrentRow.Index).Cells(0).Value
cnpel = New MySqlDataAdapter("select Titulo, Formato, Contenido from registrodepeliculas where Ean='" & ean & "';", con)
cnpel.Fill(dspel, "regpeliculas")
If dspel.Tables("regpeliculas").Rows.Count > 0 Then
filapel = dspel.Tables("regpeliculas").Rows(0)
DataGridView1.Rows(DataGridView1.CurrentRow.Index).Cells(1).Value = filapel("titulo")
DataGridView1.Rows(DataGridView1.CurrentRow.Index).Cells(2).Value = filapel("formato")
DataGridView1.Rows(DataGridView1.CurrentRow.Index).Cells(3).Value = filapel("contenido")
Else
DataGridView1.Rows(DataGridView1.CurrentRow.Index).Cells(1).Value = ""
End If
dspel.Clear()
dspel.Dispose()
cnpel.Dispose()
SendKeys.Send("{up}")
SendKeys.Send("{tab}")
SendKeys.Send("{tab}")
SendKeys.Send("{tab}")
SendKeys.Send("{tab}")
Case 4
copias = CDbl(DataGridView1.Rows(DataGridView1.CurrentRow.Index).Cells(4).Value)
If copias > 0 Then
SendKeys.Send("{up}")
SendKeys.Send("{tab}")
Else
SendKeys.Send("{up}")
End If
Case 5
precio = CDbl(DataGridView1.Rows(DataGridView1.CurrentRow.Index).Cells(4).Value)
copias = CDbl(DataGridView1.Rows(DataGridView1.CurrentRow.Index).Cells(5).Value)
If precio > 0 And copias > 0 Then
DataGridView1.Rows(DataGridView1.CurrentRow.Index).Cells(6).Value = precio * copias
For i As Integer = 0 To DataGridView1.Rows.Count - 1
vsubt = vsubt + CDbl(DataGridView1.Rows(i).Cells(6).Value)
Next
tsubtotal.Text = vsubt
tiva.Text = vsubt * 0.15
ttotal.Text = CDbl(tsubtotal.Text) + CDbl(tiva.Text)
SendKeys.Send("{up}")
SendKeys.Send("{tab}")
SendKeys.Send("{tab}")
End If
End Select
End Sub
End Class