Remove the three Phillips screws in the battery compartment near the latch. Apple was nice enough to tilt these screws at a slight angle to make them easier to remove.
Do not yank the upper case off quickly. The case is attached to the logic board via a ribbon cable.
Lift up the back of the case and work your fingers along the sides, freeing the case as you go. Once you have freed the sides, you may need to rock the case up and down to free the front of the upper case.