how do i change stop light bulb?

usually with a lot of GM cars you have to remove the tail lamp assembly which is held into place by studs and nuts that protrude into the trunk. Pull back the trunk lining behind the lamp you want to replace the bulb on and start taking any visible nuts off of any studs you find till the lamp begins to fall away from the body. After the lamps away, you'll have access to the bulb sockets.