There is typically a plastic facia that has to be removed before you can get to the screws that are holding your stereo in place.  Once the screws are removed, just unplug the rear connections and replace the deck with your favorite aftermarket unit.  Make sure to replace the factory radio with an aftermarket of about the same size (DIN2, DIN3, DIN4, etc).  This will look the best when the job is done.