Take off the oxygen sensor, unbolt the catalytic converter and slide it along the exhaust pipe until it falls off. In some vehicles, the converter is welded to the underside. In this case, put the car up on a lift before using a rotary saw to take the converter off.
Taking off a catalytic converter is only legal if you plan to replace it with a newer model. Some people want to take it off to allow the car to reach its total horsepower potential. Replacing the catalytic converter with a simple exhaust pipe or just failing to replace it can result in receiving a ticket or even having the police impound the car, depending on state law.
For those wanting to put in a replacement converter, though, start by purchasing an O2 wrench, available for around $40 from parts shops, as of 2015. This is necessary to remove the oxygen (O2) sensor. Use a socket wrench to unbolt the catalytic converter. If the catalytic converter turns out to be welded, consider having a professional handle this part, particularly the installation of the new converter, because of the metalworking involved. Don't just toss the old converter aside, as they often have precious metals lining the inside that are valuable in resale.