Depends on if it is a latch or key cylinder. If it's not keyed, it will have a hole in the center of it. I can never remember from one time to the next exactly how it works, but you stick a paper clip or similar down the hole while the thing is turned right, and it locks some little tumblers "in" so that they don't hang in grooves. There are 2, and once both are locked in, the whole center of the cylinder will pull out. I just did this about 2-3 months ago, but don't remember the exact sequence, took me about 5-10 minutes of jacking around with it to get it out. The key works similarly but the key is what holds the tabs in. If someone else doesn't have the official procedure or a good description, and you don't work it out on your own, I'll go out there and grab one of mine to figure it out again and try to post a specific set of instructions.
Too bad I'm getting old and my mind is failing... This is something you do maybe once every 5 years at most and it just doesn't stick in my mind.