You're talking about the brake light? There should be 4 screws on the brake light lens. Remove those, and the entire lens slides straight back and out. Don't twist it, try to lift it to pop it out, or anything like that, it slides straight back toward the back of the bike.
There's an o-ring type rubber seal around the edges, and it'll probably be stuck to that. Very gently pry with a flat blade screwdriver only in the direction that the lens slides. When you're putting it back together, take that rubber seal out and put a thin film of white grease on it to prevent it sticking in the future.
After the lens is out, the bulb is one of those push-and-twist deals.
I had a heck of a time the first time I tried to get mine out, so I could figure out what bulbs are in there, so I could order LED replacements... I cracked the lens. I'll order a replacement when I get a round tuit.