@Luz555
There are 3 optons:
1. The guest need to cancel the reservation and the refund (if any) will be calculated automatically, based on the cancellation policy attached.
2. The guest can sent you a change request (shortening the reservation), refund will be calculated automatically based on price difference
3. You can also shorten the reservation and as a host you can amend the calculated price in the change form to what you think is a fair deal.
I personally allways use option 3.
If there is any refund it will be set in your Transaction History and deducted from the next payout.
BTW Never cancel the reservation yourself, as you will be punished by Airbnb !