Be gentle and apply consistent, even pressure when removing the rear trim panel. There are 6 or 8 pop/friction type fasteners and they can be broken very easily. Once that's off, set it somewhere where it's out of the way from potential damage. Once that's off you'll be able to see the 6 10mm nuts holding on the spoiler, two per station. An extension for your socket, preferably a wobble-type, makes this part easy. Pay particular attention to the area around the seals at the mounting bases and make sure they're clean and free of dirt or debris before installing the replacement.
If you've got satellite radio, this is an excellent opportunity to place the antenna inside the spoiler for a slick installation.
How badly broken is the original? Would you consider parting with it?