Private Sub Button_Click

        DataGridView1.SelectedCells = InputBox("Введите значение")

End Sub

    Private Sub Button7_Click(sender As System.Object, e As System.EventArgs) Handles Button7.Click
        Dim str As String
        Dim irow, icolumn As Integer

        str = InputBox("Введите значение")

        For i As Integer = 0 To DataGridView1.SelectedCells.Count - 1
            'irow = DataGridView1.SelectedCells(i).ColumnIndex
            'icolumn = DataGridView1.SelectedCells(i).RowIndex
            'MsgBox(irow & "   " & icolumn & "   " & DataGridView1.SelectedCells.Item(i).Value)
            DataGridView1.SelectedCells.Item(i).Value = str
        Next

    End Sub