In order to change the Quantities or the Prices in a Sales Order that has been Shipped and Invoiced you first need to cancel all the issued Documents. The following instructions are provided for Sales Orders but similar steps apply for Purchase Orders.
1. In the Sales Order you wish to edit click on the Related Documents tab and open all related documents:
2. For each document click on More > Cancel Document:
3. Go back to the Sales Order you wish to edit from the Sales Order list, it will now be in the Approved Status. Revert it to Pending and it will become editable:
4. You can then change quantities, prices, or add new products in the Product Details tab of the Sales Order.