The ability to change the Project ID or account number on a PO depends on whether a payment has been made on the PO or not.
If no payments have been made, follow the step-by-step instructions outlined in this video tutorial.
If payments have been processed, follow the step-by-step instructions outlined in this video tutorial. Also review the information and instructions outlined in the Purchasing Manual: Introduction to Procurement Processes, Chapter 2: Purchasing Processes > Making Changes to a Purchase Order and send an email to the Purchasing Specialist who handled your order and request a change order to close your PO.