message your guest and tell her if she cancels within 48 hours after she booked then she will be fully refunded, including Airbnb fee. If she booked earlier then she will be refunded according to your cancelation policy. Tell her to cancel as soon as possible so you have a chance to rebook those dates.
If she doesn't cancel in a day or two and don't respond to your messages then contact Airbnb and ask them to contact your guest regarding her cancelation.