You are unlikely to be able simply modify the reservation. You will need to make a new, shorter reservation, and then cancel your originally reservation. If you want to try a an approach direct to the hotel you could phone them and see if they agree to make an exception. When you call, do not take any phone option other than 'operator' or 'any other matter'. Any option that mentions making, modifying or canceling a reservation will take you to a call center that can not help.