codice:
 Dim objConn As New OleDbConnection("Provider=Microsoft.Jet.OLEDB.4.0; " _
 + "Data Source=netguestbook.mdb")
 Dim sqlUpStr As String = "UPDATE utenti SET username = '" & txtProUser.Text & "', password = '" & txtProPwd.Text & "', mail = '" & txtProMail.Text & "' WHERE id = " & hdnUs.Value
 Dim objCmd As New OleDbCommand(sqlUpStr, objConn)
 objConn.Open()
 objCmd.ExecuteNonQuery()

ke errore c'è?