Computer Store Project - Working with DataGrid

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VB

VS2012

I have this project to do working with database in viusal basic, but the problem is i have the code its correct i mean it doesn't come out as error but it doesnt function when running the program

Here is the problem:
this is my code for the update button( to update the data in the datagrid)

Private Sub btnUpdateComp_Click(sender As Object, e As EventArgs) Handles btnUpdateComp.Click
    Dim strTheComputerSelected = DataGridView1.SelectedCells.ToString

    Dim theComputer = (From aComputer In DB.Computers
                     Where aComputer.SerialNumber = DataGridView1.Focused.ToString
                     Select aComputer).FirstOrDefault



    theComputer.SerialNumber = txtSerialNum.Text
    theComputer.Color = cboColor.Text
    theComputer.ManCompany = cboMCompany.Text
    theComputer.OpertionSys = cboOS.Text
    theComputer.Price = txtPrice.Text
    theComputer.RAM = txtRAM.Text
    theComputer.ScreenSize = txtScreen.Text
    theComputer.Type = cboType.Text
    theComputer.Weight = txtWeight.Text
    DB.SaveChanges()
    UpdateComputersList()
End Sub

but it does not work!!

Any Comments?