Assuming you are referring to this type of light fixture...
The lens should slide AWAY from the center switch about 3/8" to 1/2" and then drop down and away from the fixture.
I just squeeze in on the lens and slide away from the center of the light fixture and it falls into my hand... There is also a small clip on the top of the lens you may have to push in on at the same time you squeeze the two sides..
You can almost see the four slots the lens uses to hold itself inplace inthe above photo...
Once you do it you will see how easy they comes off...
Roy Ken