If you haven't received you can click the unissued button then modify it and issue it again. Check your dates. You may wish to take a screen shot so you can keep what you originally had before you modify it. If you have marked items as received you will need to reverse the quantities by entering a negative number as received, save it and go and unissue it. Make your changes save it and then go back in and show the items you previously received so you can close the PO if you wish.