Get in the back seat of the car.
The housing for your brake light is the gray plastic thing. The “shelf” is the gray fuzzy flat thing it’s sitting on. There are 4 hooks holding the assembly onto the shelf– 3 on the shiny side (facing outward), 1 on the dull side (facing you). See the link below for two pictures of the hooks on the housing so you know what you’re working with.
Push firmly against the dull side, pushing it toward the window. While you’re doing this, either wiggle it or try to lift the dull side to unfasten the single hook on that side. Once you’ve succeeded there will be a quarter-inch gap between the shelf and the housing.
Pull gently on the housing to release the remaining 3. Don’t pull too hard– there isn’t much slack in the wiring so you could rip it straight out if you’re not careful. Also be mindful of your rear window; hitting it with the brake light assembly could damage your defroster. Also, if you shatter the bulb it’s a pain to clean up and remove from the wiring harness.
Once it’s off completely, you’ll see the wiring go into the housing through a little black thing. That’s the wiring harness. Turn it a quarter-turn and pull it out of the housing. Don’t force it– try turning it the other way if it won’t budge. Be gentle with it to avoid breaking the bulb.
With the wiring harness extracted, remove the light bulb like you would a tooth– wiggling while firmly pulling. It does not screw in. Stick the new bulb in and work your way backwards through the steps above.