Welcome to Forums.. As new member, you should continue practicing using the "Search"tab above in blue bar.. Try clicking on it, and typing in words with a space between like: light fixture bulb change and see what you get back...
As for your specific, you don't say whther unit is flourescent or small incandescent bulbs.. Most of the plastic covers have tabs (breakable if plastic gets brittle..) that can be released by pressing in from one or two edges.. Without a picture or better description, it is hard to offer more specific advice...
If you still had owner manual, the process is described there for your fixtures.. If not, you can search here ( owner manual replacement ) and find two sources of replacement manuals, which are handy things to have around..
In Theory, there's no difference between Theory and Practice, but in Practice, there is usually a difference...