When she cancels the cancellation policy decides about a refund (if any).
So it is done automatically. You can see in your Transaction History (future transactions) the amount you will receive. If the guest wants more refund, she should contact Airbnb. If you want to give some refund, then it will be deducted from this upcoming payout regarding this reservation (you receive less). If payout is already processed, it wll be deducted from another future payout.