How to increase cell quantity using keyboard up and down arrows ?

Question

How to increase cell quantity using keyboard up and down arrows ?

 

I work on windows form desktop application using csharp

 

I face issue I can't increase quantity or decrease  using press on key up or down

on datagridview control 

when have quantity 1 value then when press up arrow then will be 2

when have quantity 2 then when press down arrow will be 1

 

what i try

private void GridTrxInvF_KeyPress(object sender, KeyPressEventArgs e)
    {
        int currentNumber = Utilities.ObjectConverter.ConvertToInteger(GridTrxInvF.Rows[GridTrxInvF.CurrentCell.RowIndex].Cells["Quantity"].Value);

        if (e.KeyChar ==(char) Keys.Down)
        {

            GridTrxInvF.Rows[GridTrxInvF.CurrentCell.RowIndex].Cells["Quantity"].Value = (currentNumber - 1).ToString();
        }
        else if (e.KeyChar == (char)Keys.Up)
        {
            GridTrxInvF.Rows[GridTrxInvF.CurrentCell.RowIndex].Cells["Quantity"].Value = (currentNumber + 1).ToString();
        }

    }

 when press up arrow or down

it moving to quanityt previous row  when press down keyboard button

or moving to quantity Next rows when press up keyboard button

so what i do to solve issue please ?