You go to "my reservations", find the relevant booking, choose "change or cancel" and then enter the new dates. A message will be sent to the guest for him/her to accept. Once accepted you will get paid for the correct number of days.
Ask your guest to request additional days and your will grant them. Happpened to me ones and everything went smoothly ... when you get the request, just approve it and you will have that revenue added in your payout. Very simple procedure.