Well, the short answer is you can't unless you are the Approver/Account Manager/Purchaser and it is at that level.

What you can do is cancel the order (if it isn't complete), duplicate it. Then resubmit the duplicated order.

You may also add a note indicating what edit you would like, but there are no guarantees an approver will make the change for you.

Note, a completed order in PrePurchasing cannot be edited except to add notes, add attachments, and receive line items.