How do you change BOTH the front and rear speakers on a 1992 Geo Prizm and is it possible to install larger speakers?

I recomend you turn off the car completely with the key off the ignition. Front speakers can be accesed by pulling off the door panel in most cases and taking off a couple of screws holding the speakers onto the panel. The panel itself might be screwed onto the door using a screw around the opening handle and sometimes more screws towards the bottom. Be careful never to yank any wires as you pull the panels and speakers off the car! Rear speakers can be accessed from the rear seat towards the rear windshield and on some models from the trunk. The speakers are being held again by a couple of screws. Mostly, you cannot install bigger than factory size speakers because the fitment on the doors and the rear trunk area just will not let the larger speakers sit comfortably to where you can put the fastening screws back on. Good luck.