On square ones some times the lens tabs into the housing on two opposing sides. You have to push one of these sides away from the housing to release the tabs.
We need more clues.(picture)
BTW you need three posts under your belt before you can add pictures.
The brass, round dome light has 1 toggle but no visable screws. Need to get inside to change bulb. Have not been able to turn the outer casing and fearful of damage by prying
Use a small head screw driver or thin knife and pry inner ring or the ring around the lense. It just snaps in and it should come out enough to where you can grab it with your hand giving you access to the bulb. Same ones I have.