IF you're referring to the typical ceiling mounted square lights and lenses, gently squeezing on opposite sides of the lens, while gently pulling downward should pull it free - after doing it once, and seeing the method used to retain the lenses, it will all seem very easy...
John Day....|'88 Winnebago Super Chief 27ft. Class A
Eastern .....|'88 KIT model 240 24 ft. 5er
Oregon ......|'02 Dodge/Cummins 2500 Quad Cab