A Purchase Order transaction may be edited if it has not been confirmed and completed.
The procedure is as follows:

3. Click to to highlight the appropriate Purchase Order transaction. (It will either be showing as Planned or Confirmed).
4. Select [F3].
5. Edit details as required, by clicking on the appropriate box to change, or Add or Delete lines as necessary.
6. Process [F9], and select options as already set out in the section "Entering a new Purchase Order".