@Paula1957
There is no policy, but you probably have to compensate the guest for the extra costs of staying in another accommodation if your listing is not available for the guest. Or cancel the reservation (penalties will be applied by Airbnb), but guest will receive full refund and you are "out of the contract".
If reservation has already started maybe shortening the reservation ("change" option) is an option (full refund of unused nights, or more if you amend the price in the change-form). But guest must accept the "deal".