how to change value of cell of datagridview at run time after executing ?

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C#

i want to change price of amount at run time
for example

Amount=Rate*Qty suppose answer is 25
but at run time i want to change amount 25 to 26 please help me what should i do? beacuse price is in dollar
my code is given below

private void dgDetail_CellEndEdit(object sender, DataGridViewCellEventArgs e)
      {



          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());
                              }
                          }
                      }

                      //}


                  }


              }


        // }
      }