To replace a catalytic converter, lift the car, remove the old catalytic converter from the car, install the new converter and lower the car. Buy a new converter that is compatible with your car, and read the user manual before attempting this task.
Start by jacking up the car to allow for better access to the catalytic converter. Locate the bolts that secure the converter in place, and loosen them off with a wrench. For hard-to-remove bolts due to corrosion, apply a drill or a saw to remove them. Take off the nuts and seals as well.
Once the bolts, nuts and seals are off, remove the oxygen sensor with an oxygen wrench, and take off the connection pipe between the sensor and the converter, referring to the manual as needed to determine the exact number of pipes that need removal. Then remove the old catalytic converter from the muffler, and set it on a protected surface. Place the new converter into the space previously occupied by the old converter, and secure it in place with bolts. Replace the pipes that connect the converter to the oxygen sensor, taking care to replace the damaged pipes with new ones. Finally, lower the car.