@Patricia3993
Do NOT accept a reservation alteration for this guest. He will need to cancel the stay if leaving early and as @Helen3 says, he will be refunded depending on the circumstances of the remain days he cancelled and your cancellation policy for long term stays.
Airbnb applies a different cancellation policy for stays 28+days than your standard cancellation policy. Hopefully you set that when creating your listing:
Click the Cancellation Policy square on the Listing Editor and another window will appear on the right showing your standard cancellation policy (less than 28days) and what you have set for 28+ days:



Lots of pitfalls with Long Term stays. Here is a Guide explaining all that:
https://community.withairbnb.com/t5/Community-Guides/Monthly-Stays-on-Airbnb-Guide-Revised/m-p/20665...