If you have an invoice with an incorrect PO#, and the job has been closed and invoiced, you can still get it corrected and reprint the invoice with new PO#.
You just need to Reopen the job, correct the PO# and save the job. You can reprint the invoice and it will have the corrected PO#.
Be sure to add a note to the job indicating what you did because they system will change the last ticket from a complete return to a partial return, and you cannot change that back. You also need to enter in the Date Completed (last ticket date) and change the job status back to Invoiced.
If the job has not been closed and invoiced, you can simply change the PO#, save the job, and reprint the invoice.