To void a receiver you would go to the Purchasing Menu / Transfers and Receipts Menu / Receipts by PO #.  Enter your PO # and a list of all receivers should populate.  There is a list of options above the receivers list - locate the # that corresponds to delete and enter that number next to any line of the receiver that you would like to delete. You can only delete one line at a time.

If this PO is for parts and they have already been removed from your inventory via a WO or SO you will either have to receive your new PO first or do a blind receiver.  You cannot delete inventory that is not there any longer.

Your incorrect PO should now be open for Maintenance to delete.