how to make cell editable in datagridview ?

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C#

i am working on inventory , i want to change my amount on run time mean

Rate* Quantiy=Amount
so i want to change my amount on run time after multiplying , pleas giude me as son as possible
my code is given below

if (dgDetail.CurrentRow != null)
{
    if (dgDetail.CurrentRow.Cells["Qty"].Value != null && dgDetail.CurrentRow.Cells["Rate"].Value != null)
    {
        if (dgDetail.CurrentRow.Cells["Qty"].Value != null && dgDetail.CurrentRow.Cells["Qty"].Value.ToString().Trim() != "" && Convert.ToDecimal(dgDetail.CurrentRow.Cells["Qty"].Value) > 0)
        {
            decimal GAmount = Convert.ToDecimal(dgDetail.CurrentRow.Cells["Rate"].Value.ToString()) * Convert.ToDecimal(dgDetail.CurrentRow.Cells["Qty"].Value.ToString());
            dgDetail.CurrentRow.Cells["Amount"].Value = Convert.ToDecimal(GAmount);



            if (dgDetail.CurrentRow.Cells["STPercent"].Value != null)
            {
                dgDetail.CurrentRow.Cells["SaleTax"].Value = GAmount / 100 * Convert.ToDecimal(dgDetail.CurrentRow.Cells["STPercent"].Value.ToString());




                if (dgDetail.CurrentRow.Cells["OtherSTP"].Value != null)
                {
                    dgDetail.CurrentRow.Cells["OtherST"].Value = GAmount / 100 * Convert.ToDecimal(dgDetail.CurrentRow.Cells["OtherSTP"].Value.ToString());

                    dgDetail.CurrentRow.Cells["Total"].Value = GAmount + Convert.ToDecimal(dgDetail.CurrentRow.Cells["SaleTax"].Value.ToString())+Convert.ToDecimal(dgDetail.CurrentRow.Cells["OtherST"].Value.ToString());
                }
            }
        }



    }