Fixing a PO When the Incorrect Quantity is Received
When you receive a PO, for each line received, it creates a PO Inventory Movement (POIM) (refer to related list on the PO Line).
To correct your mistake, navigate to the PO line that has the mistake and delete or edit the incorrect Purchase Order Inventory Movement. That un-receives the items.
When you navigate back to the PO header, you will see on the PO line that the quantity received is now back to zero (if you deleted the POIM) or to whatever number you edited on the POIM.