Replacing a 2d array value with a new value

[Sliders]

What I want to do is when a user selects a radio button on a 2nd form, it will correspond with the first form.

Example

. . .
. . .
. . .
. . .

Say I want to change the last column and the last rows number with the letter V as so:

. . .
. . .
. . .
. . V

I cannot find any documentation on how to preform this feet, this is what I have been trying so far, but all I get is either null errors or some other thing.

Public Class rad
    Dim uno, LC, R, V As String
    Dim check As String

    Private Sub btn_CheckedChanged(ByVal sender As System.Object, _
    ByVal e As System.EventArgs) Handles _
    btnUno.CheckedChanged, radLC.CheckedChanged, _
    radReg.CheckedChanged, radVeg.CheckedChanged

        If btnUno.Checked = True Then

            ''''If Chart.Then Then
            'End If
            Chart.txtA.Text = "."
            Dim a As String = "."
            'Chart.listSeat.SelectedItem = "."
            Chart.listSeat.Items.Add(CStr(seat(row, col)))
        ElseIf radLC.Checked = True Then
            Chart.listSeat.SelectedItem = "L"
            If radLC.Checked = Chart.txtA.Text Then
                'Array.IndexOf(seat) = "L"
            End If
        ElseIf radReg.Checked = True Then
            Chart.listSeat.SelectedItem = "R"
            Chart.txtA.Text = "R"
        ElseIf radVeg.Checked = True Then
            Chart.listSeat.SelectedItem = "V"
            Chart.txtA.Text = "V"
        End If
    End Sub
End Class

I was playing around with some things, but it just shows how inexperienced I am with some of the advanced features of VB.