Qty difference in PO and Vendor invoice

 Hi
We raised a PO on a vendor ordering 22 nos of a certain equipment for certain price
The po was approved and sent to the vendor. Later it was communicated to the vendor that on 21 nos are required instead of 22

The vendor sent invoice for 21

Questions

1. Do we need to cancel the PO and re enter another PO
2. Is it possible to modify the existing PO without cancelling the  same


Thanks
Ninad