Turn off the light.
Press upward on the cover, pushing it slightly into the fixture. If the cover slides around, slide it to one side of the fixture and gently tilt the other edge of the cover and pull the entire cover out of the fixture. If the cover does not push upward, proceed to Step 3.
Insert the tip of a flat-head screwdriver into an edge where the cover meets the fixture casing. Carefully and lightly pry the cover off the fixture. Note that some fixtures have spring clips holding the cover in place; prying gently on one side will release the clip. If the fixture does not release the cover, proceed to Step 4.
Inspect the fixture itself. look for clips or screws. In some cases the cover may be connected to the fixture itself. Remove the clips by lifting or sliding them out; use a Phillips screwdriver to remove holding screws.