Установить нужное значение в DataGridViewComboBoxColumn

ComboBox1.SelectedIndex = 3

Mat_Name.DataGridView.Item(0, 0).Value = 1 ' Mat_Name - название комбобокс столбца в datagridview

Private Sub Button1_Click(sender As System.Object, e As System.EventArgs) Handles Button1.Click
    With DataGridView1
        .Rows.Add(New DataGridViewRow)
        .Rows(.Rows.Count - 2).Cells(0).Value = "124"
        .Rows.Add(New DataGridViewRow)
        .Rows(.Rows.Count - 2).Cells(0).Value = "254"
        .Rows.Add(New DataGridViewRow)
        .Rows(.Rows.Count - 2).Cells(0).Value = "235"
        .Rows.Add(New DataGridViewRow)
        .Rows(.Rows.Count - 2).Cells(0).Value = "453"
    End With
    updateComboBoxCells(1, 2)
End Sub
Private Sub updateComboBoxCells(ByVal col As Integer, ByVal indComboBox As Integer)
    For Each rr As DataGridViewRow In DataGridView1.Rows
        If rr.Index < DataGridView1.Rows.Count - 1 Then
            Dim cb As DataGridViewComboBoxCell = rr.Cells(col)
            rr.Cells(col).Value = cb.Items(indComboBox)
        End If
    Next
End Sub

Dim cb As DataGridViewComboBoxCell = rr.Cells(col)
rr.Cells(col).Value = cb.Items(indComboBox)

        Dim cb As DataGridViewComboBoxCell
        If LaminateGrid.CurrentCell.ColumnIndex = 1 Then
            For Each dr In Me.Mat_GridView.Rows
                i += 1
 
                If dr.Cells(0).Value <> "" Then
                    If i = Me.LaminateGrid.Rows(Me.LaminateGrid.CurrentCell.RowIndex()).Cells(1).Value Then
                        cb = Me.LaminateGrid.Rows(Me.LaminateGrid.CurrentCell.RowIndex()).Cells(0)
                        Me.LaminateGrid.Rows(Me.LaminateGrid.CurrentCell.RowIndex()).Cells(0).Value = cb.Items(i - 1)
                        Exit Sub
                    End If
                End If
            Next
        End If

i += 1

dr.Index