If you want to change the dates of a booking you've confirmed, you need to coordinate your decision with the owner of the apartment.
In case you want to shorten the stay of a confirmed booking, please remember about the cancellation policy. Most often the changes that mean shortening the stay are possible only 7 days and more before the check-in. If you're already staying in the apartment, shortening the stay is possible only with a fee of the price for 1 night stay as a penalty for cancelling one or a few nights of stay.
If you want to prolong the stay, make sure to contact the owner as soon as possible to talk the changes through. If the owners have an opportunity to prolong your stay, they will gladly do it.