Moderate cp:
If the guest arrives and decides to leave early, 50% of the accommodation fees for the nights not spent 24 hours after the cancellation occurs are refunded.
Cleaning fees are always refunded if the reservation is canceled BEFORE check-in.
You don't have to do anything, it is an automated process and Airbnb will handle it. Some hosts will offer to refund for the days they will be able to rebook, but it is their goodwill.
If you will feel bad anytime your guests cancel and have to pay for the service they didn't use then the best would be to switch to flexible cancelation policy.