Assuming you need to change the brake light on the taillight assembly, here are instructions for a 2008 Mitsubishi Outlander.
1. Open the trunk and prop it open.
2. On the back of the taillight assembly, there are two screws holding it in place. Remove these screws with a Phillips head screwdriver.
3. Gently pull the taillight assembly away from the car body far enough so that you can reach behind it and access the brake lightbulb socket.
4. Turn the bulb socket counterclockwise to loosen and remove it from the housing. Pull out the old brake lightbulb and insert a new one in its place (make sure to get one with the same wattage).
5. Reinsert bulb socket into housing and turn clockwise to tighten. Push taillight back into place against car body and replace screws to secure it in place