@Angela2072
The guest will have to contact Airbnb themselves, explain that the host (=you) agrees to a full refund *for the stay*, then Airbnb will check messages exchanged between you and the guest to confirm that you agree to a full refund. Someone from Airbnb may check back with you again as well to make sure you agree with the cancellation and refund. Then Airbnb will handle the cancellation and refund whatever is due to the guest.
You should make it clear to the guest that hosts have no control over SERVICE FEES, and it may not be refunded. The host can only decide to refund the payout that would have been paid (to you) for the stay. Airbnb has the final say and power to decide whether they are willing to refund their own fees to the guest.