It is very easy for a guest to amend his booking dates @Anonymous . All he has to do is go into the booking and hit the change/alter button and put in his new dates. Then you will get a message to confirm or not accept the request.
Or you can go into the booking and do it and the guest will need to accept the change.
Have a look at Airbnb Help under 'change a booking' and it gives you instructions on how to do it.