The best time to do this is during a brake job. This applies to both front and back rotors. If you are doing the back rotors, make sure you don't have the emergency brake on or you'll never (and I do mean never) get them off without serious damage to something.
Remove the tires.Using an Allen wrench/socket remove the brake calipers.Using a wrench/socket remove the frame that holds the calipers.If this is the first time you're doing rotors you may have 2 or 3 of these round things (sorry, the name escapes me) that hold the rotor on at the factory. Take a screwdriver and some pliers and unscrew them off the threads. You do not need to keep these unless you really want to.The rotors will just slide off now. If they're stuck at all, get a mallet (not a hammer) and tap around the backside of the rotor to loosen it up.
Good Luck.