You go to Your Reservations and you will see their booking - click on Cancel/Change and you will be allowed to enter the new pricing. Include all accommodation fees, cleaning fees, additional fees agreed such as the pool heating, but DO NOT inlcude AirBnB service charges. These will be applied automotically. Ensure your guest knows that there is a revised cost on its way and ensure that they click on it to accept, otherwise you will only receive the original amount