Open the 
Posted Purchase
Receipt window and browse to the relevant document.
Select the line or lines you want to correct and click Functions, Undo Receipt.
If you have made an incorrect quantity posting, that is if you have made a purchase order with, for example, the wrong number of items and posted it as received (but not invoiced), you can undo the posting.
Open the Posted Purchase
Receipt window and browse to the relevant document.
Select the line or lines you want to correct and click Functions, Undo Receipt.
The program will now insert a corrective line in the posted document and the Quantity Received and Qty. Rcd. Not Invoiced fields on the purchase order are set to zero. If the quantity was received in a warehouse receipt, the program will insert a corrective line in the posted warehouse receipt and, if the quantity was only partially received, update the Qty. to Receive on the warehouse receipt.
Go back to the purchase order and click Functions, Reopen to reopen it.
Correct the entry in the Quantity field and post the order.