Reverse of GR for some qty in a line item

Dear Experts,

Could anyone please help me. I need to cancel GR of some quantity for a line in PO. For example: in a PO line item 10 there were 4000 qty and all 4000 was received. and then 2000 qty issued to network against a reservation. At first 4000 GR was wrong, it should have been 2000. Now i want to reverse the remaining 2000 qty.

By what process i can do that, pls advise. It will be return delivery.

Thanks and Regards,

Zusen