Change Requested Delivery Date on PO PDF

We have a PO that we need to change the "Requested Delivery Date" on the PDF document.  The PO has two lines and the first line has already been cancelled.  The need by date was changed on the 2nd line, but that doesn't seem to affect the need_by_date/requested delivery date that shows up on the PDF.  The need by date on the line that was NOT cancelled was changed, but that doesn't seem to help.  How can that date be updated at this point?  Thanks.