How to edit an open order
First you need to go to the POS module.
After that you want to find on the top menu where it says Orders. Now you want to click on Open as you see it on the picture.
You can see all the open orders that you have. Find the one you want to edit and click on it. Now you want to click the edit button.
After that you can edit the order by adding or deducting a products quantity or by totally deleting the product from the order.