You can't, not in any version

go into it and add more lines.  a PO is only "complete" if all it's lines are complete. It doesn't stop you from adding more afterwards.

Hi R.K as Jay says you can't outright re-open it and if you need to remove things from the PO you're out of luck.  As Mike Cook says you can add additional lines to the PO and re-post it but this won't help you if you need to reduce quantities, so the answer is two fold:  If you can add items to the P/O you can do this with insert and re-open it.  If you need to do anything else as Jay says you can't and need to move onto a new PO.

if it had the ability to "reduce" items, then it wouldn't be complete.  and you can't delete lines if there is an RC posted against the PO.

Thank you, well said!