the majority of them I've dealt with of them are just pressed in to the aluminum plate.... you can either pry it out like has been suggested, or remove the plate from the wall and push it out....
some of the older plates had plastic grommets/bezels that held the light into the plate, and if they break you probably wont be able to find a replacement....
but there are options... some electronic stores, where you find small LED replacement lights, they usually carry the special rubber grommets to hold the lights in, but if not, a properly sized grommet from the wiring section of any hardware store will work.
in either case you may have to drill out the hole in the plate a little to get the new rubber grommet to fit, but it will look nice and work great.