How can I update quantity on an open internal requisition line after it has been approved

Hi,

 I am trying to update the quantity on an internal requisition line. The requisition has been approved and there is an internal sales order created for it. I am able to update the quantity on the internal sales order but I am not able to update it on the requisition line. I created the requisition line for qty 10 originally but need to change the qty to 5 now.

This question is for Oracle 11.5.10 release.

Thanks!