How do you correct a voucher and receipt that have the incorrect quantity?

A PO was created for quantity of 4000.  2 receipts were created against this PO for quantity of 800 each and paid successfully.  A third receipt was incorrectly created for quantity 2400 and voucher was created for this receipt.  This matches the PO quantity now and the PO can no longer have receipts created against it.  How can I correct the quantity amounts on the paid voucher and receipt(should be 800 also) so the PO can be used for the remaining 2 payments of 800 quantity?