How to change the text of DataGridViewTextBox column programmatically?

Question

I have a DataGridView of type 'DataGridViewTextBoxColumn' in which following fields are there :

SrNo. |    Description   |    HSNCode    |    Qty    | Rate    | Amount  

I want allow the user to enter the item Description , Qty and Rate

The "SrNo." column is read only I want to generate the SrNo. automatically as the user enters to "Description" cell i.e. I want to display the row index in that cell.

And also I want to calculate the "Amount" (Qty*Rate) in my program as user leaves the "Qty" field.

I tried the following code :

private void grdData_CellValueChanged(object sender, DataGridViewCellEventArgs e)
{

    if (e.RowIndex >= 0)
    {
        if (e.ColumnIndex == 0)
        {
            grdData.Rows[e.RowIndex].Cells[0].Value = e.RowIndex.ToString();

        }
        else if (e.ColumnIndex == 5)
        {
            grdData.Rows[e.RowIndex].Cells[5].Value = 
            ((Convert.ToDouble(grdData.Rows[e.RowIndex].Cells[3])) *    
            (Convert.ToDouble(grdData.Rows[e.RowIndex].Cells[4]));
        }
    }
}

How to do so?