Adding PO# to an AR invoice already printed

Sage 100 v2018 - trying to add a missing PO# to an AR invoice already printed via: 

  • AR Inv entry 
  • inv # 
  •  cust # > added PO#. 

No other changes are to be made on inv.    Print invoice -.  .  .  I continue to update Sales Journal and daily register states nothing in file.    PO did not poplate on invoice.  Data not selected (nor do I have a box for already printed invoices)  HELP.