@Ronnie123
Your question is in the title of the post and in the body is a huge empty space.....
Answer:
- The guest can sent you a shortening request ("change" option on the reservation)
OR
- The guest can cancel the reservation
In both cases the cancellation policy will automatically calculate the refund (if any)
You as a host can also use the "change" option to shorten the reservation, having the option in the change form to override the calculated price by amending it