trying to replace back lens on tail light

I parked my hubbies' Prius in our driveway and someone knocked out the back tail light cover. I bought a new cover. I want to remove the old cracked one. I opened the back panel and there are these little nobs sticking out around the outside edge of the light casing. I am guessing i need to push these out or pinch them and pull them off or something to that effect to get the lens cover off. The lights work fine so I ONLY need the cover changed out.Has anyone out there done this themselves yet? Any advice? I am very good with working on cars and have done it a lot. These I have never seen before and don't want to mess up the car. Also I don't want to pay the dealer to pop the lens off either... help! Ganesha's Wife