Problem
You created a PO on the wrong job and it is now in received status. You need to cancel the PO and move it to a different job.
Solution
Go to the PO on the invoice.
Under the Documents section, click Receipt.
Under the Action tab, click Cancel.
Once the receipt is canceled the PO is placed back into Sent status and then can be edited by clicking the Action tab > Edit and the proper job number can be assigned.