Summarize this article in one sentence.
The oxygen sensors are small metal pieces attached to wires that connect into the sides of the exhaust pipe. Place a wrench around the bolt connecting the sensor to the pipe and rotate it counterclockwise until it's loose. Pull the oxygen sensor straight out from the pipe before unplugging it from the wires connecting to your vehicle. Your vehicle will usually have 12 oxygen sensors connected to the exhaust, but it may vary between make and model. Vehicles only have oxygen sensors if they have a catalytic converter. If your vehicle is from 1974 or prior, then the exhaust may not have oxygen sensors. Lie on a rolling creeper trolley to move around under your vehicle more easily. You can buy a creeper trolley from an automotive supply store. Wear clothes that you don't mind getting dirty as well as disposable gloves so you don't get too greasy. Penetrating fluid works between the bolt and nut threadings to add lubrication and make it easier to unscrew the system. Point the spout of the penetrating fluid directly at the nuts connecting the exhaust pipe to the downpipe attached to your engine. Make sure to coat all of the threadings evenly to make the job easier. You can buy penetrating fluid from your local hardware or automotive care store. If you don't have any penetrating fluid, it may be difficult to remove your exhaust system on your own. If your exhaust system is made up of multiple pieces, then look for bolts where the pieces connect to one another. Spray those with your penetrating fluid as well so you can remove each of the pieces separately. Start where your exhaust pipe connects to the engine at the front of your vehicle. Choose a ratchet bit that matches the size of the nuts holding the exhaust system to your vehicle. Place the end of the ratchet over the nut you're loosening and turn it counterclockwise. Apply firm pressure as you rotate the nut since it may be stuck or locked in tightly. Continue unscrewing the rest of the nuts if the pieces of your exhaust system are held together by other bolts or clamps. Use a ratchet with a long handle so you can get more leverage while you're disconnecting the old exhaust system. Look near the top of the pipe along its entire length to find the metal pins that extend up and into rubber rings on your vehicle's body. Support the exhaust pipe with your nondominant hand as you slide the metal pin out from the rubber. Slowly and carefully take the other pins off so the exhaust system comes loose from the body of your vehicle. If you have trouble pulling the metal pins off of the rings, try lubricating them with soapy water to help them slide out more easily. Don't let the exhaust pipe drop since it could be heavy and it may hurt you or damage other components under your vehicle. Slowly take the exhaust system from the catalytic converter to the tailpipe and lower it to the ground. Carefully remove the system out from underneath the vehicle so it's out of the way while you're working. You can either throw the old exhaust system away or you can try to sell the parts if they're in usable condition. Do not drive your vehicle while you have the exhaust system removed since it may be illegal in your area and you will release harmful fumes.

Summary:
Disconnect the oxygen sensors from underneath your vehicle. Spray the nuts along the exhaust pipe with penetrating fluid. Unscrew the nuts with a ratchet. Unhook the pipes from the rubber exhaust hangers. Pull the exhaust system down from your vehicle.