Approved Purchase Order is a company's procurement history.
If this Order contains some error you're able to fix it up.
1. Find the Order and press Revise
The Order is available for the Revise until it contains some not billed and/or not received products.
2. While revising the Order you can edit products, add or delete/reject something.
3. Confirm the Order when you are ready. The Order will be sent for approval again.
If you didn't receive some items and the supplier isn't going to bring them — you can reject these items in PO and after that close the PO.