Suspended Ceiling Lighting

If this is the standard 2x4 or 2x2 box lights up there are two latches on the long side of a 2x4 or on one side of the 2x2.
Gates, either pull down or in most cases 2x2, they are spring and push one side

There are so many types of lighting so that it is difficult to answer. Is it a mount or surface? If it is recessed and you can not see any obvious way to remove the lid and you could raise a ceiling panel next to it and you should see some clips inside for holding the fixture in place . Undo and drop it down for a closer look.
If it is surface mounted so you should carefully consider the edges of the installation and you should find a clip. Still having problems? take a picture and upload flckr or something that we can have a look!

There are many different kinds.

The first three are probably

1. If it is a type of flat plastic sitting in the fixture. Just push and slide along in the light of the ceiling tile next. I do not take these out right if I can help because it is the most delicate and easily broken.
2. Silver search sorting. If it looks like he sits in the fixture, lift one side, you should be able to move a few mm to decrease the hole.
3. If it is outside of the fixture, appears to be pushed toward the ceiling. They are often held by spring pressure. just pull down. Come by hand will not need to force. The look for spring on the side. Squeeze the spring and to release him from his home.

That's about the main but there are loads of strange and wonderful arrangements there.

tc hasn't a clue, there are as many types it is impossible to say, but they are all easy to take uot more lift and drag, or simply lift and tilt, but if your elevator blocked tileout next to the light, and lift up the light assembly which will allow access to the defuser, hope this helt

All right.

First, get an OSHA approved ladder and see what tools you need to remove it.
Second, use the tools at hand to remove the lid of the lantern.