@Jim2223 Firstly DO NOT CANCEL - It will cause you all sorts of grief/expense.
Best way is to message the guest saying that they need to call Airbnb and ask for a cancellation/refund and that you will then agree to a refund. In all likelihood support will then call you and see if you agree to a refund at which point the guest will also get Airbnb's fees back.
If you cancel the guest will be fully refunded but you will be fined and your cancellation rate will stop you becoming a superhost for a year.