From the transaction window you can change the quantity of an already entered item by pressing Change Qty on the bottom of the screen, or hitting the F9 key.
A Change Quantity window will pop up, just enter the desired quantity of your item into the New Qty window.
Once you have entered your desired quantity, press the Make Changes key to make your changes.
If you have switch 055 enabled, you can change the quantity here.
You can now continue with your modified transaction.