depending on the year/style, you'll either need a philips or a flathead screwdriver.
one style uses the outer beziel to lock the plastic cover in place. generaly speaking, there're 2 philips head screws that you unscrew to make it pop off. the other style, uses a push in plastic cover, that sticks in itself. this generaly can be pulled, by sticking a flathead screwdriver in along the edge of the cover, which'll cause the tang to disengage from the housing.
kinda confusing, but its the end of the day here at work, and my brains' all fried.
-Ddragggon