If it has the 16" chromed wheel covers then it's held on by the lug nuts. Remove the center cap, jack the car up a little bit and take the nuts off; they have a collar around them that retains the cover. Once they're off the cover should pop off with some gentle persuasion from a flat head screw driver. If it has the old style covers with the "wired" look around the rim, they just snap off of the wheel edge with a little prying.
Bolt on:
Snap on: