@Joe761 If you're happy to have him reschedule, you can go to the reservation and click Change or cancel reservation, then modify the dates to what he's requested. He can also go in and do it himself. If you do it, he will have to approve the date change: if he does it, you will need to approve it.
Please be aware, though, that this is a common way that guests get sneaky refunds. They will change the dates so it's outside of your paid cancellation window, then cancel anyways. Again, as long as you're okay with that possibility, change away!