how do I change a brake light bulb?

You get at from the trunk. Look on youtube I have found all sorts of videos there. Perhaps go to an auto parts store when they are not busy, ask the parts person for the correct bulb and ask if they would mind showing you how to change it.

On the older Buick Centurys you have to open the trunk and remove 6 or 8 black wing nuts to pull away the wide tail lamp assembly. Careful not to damage the wires or the lamp holders. The lampholders unscrew from their sockets allowing bulb replacement. I believe there are 16 lamps including the back up lamps. Turn the lights on and put a piece of tape or mark the bulbs that are bad. Turn the lights off and start replacing the bad lights.