The air conditioner belt on most cars is on a separate belt and pulley from the serpentine belt that drives all of the other devices. Because of this, changing the air conditioner belt requires removing the serpentine belt first before the air conditioner belt can be removed and replaced. This inner belt might be a serpentine style belt if it has to do more than run the air conditioner. But most of the time, it is a V-belt that runs in the groove on the pulley.
Make sure the car engine is off. Raise the hood. Locate the belt tensioner. This a small pulley usually located below and off to one side of the alternator. Use a ratchet and socket to turn the nut on the tensioner pulley. This will move the pulley and let tension off of the serpentine belt so you can remove it. On most cars, you will turn the nut clockwise to relieve the pressure. The socket size will typically be from 1/2 inch to 5/8 inch or 13 to 15 if it is metric, depending on the model of vehicle.
Slip the loosened serpentine belt off of the tensioner pulley by sliding it to the outside of the pulley. Slip the belt off of the remaining pulleys and lift it up and away from the engine and out of the car. Set it to the side so it can be replaced onto the vehicle later.
Loosen the adjustment bolt on the air conditioner bracket, using a ratchet and socket or wrench. When enough tension has been reduced on the belt, lift it out of the pulley and off of the car.
Slide the new air conditioner belt into position on the air conditioner pulley and around the crankshaft pulley. Use the socket and ratchet or wrench to tighten the adjustment bolt to restore the tension to the new belt. The belt should be tight enough that it does not move easily when pushed down or pulled up.
Lower the serpentine belt back into the vehicle. Make sure the grooves on the underside of the belt fit into the notches on the various pulleys it drives. A diagram showing the circuit of this belt is located near the hood latch at the front of most vehicles. The last pulley will be the tensioner pulley. Place the socket and ratchet onto the nut on the pulley. Let the belt rest on the socket as you turn the nut clockwise. When the tensioner is fully rotated, ease the belt onto the pulley and release the tensioner to tighten the belt.
Start the vehicle and turn on the air conditioner. If the air conditioner belt squeals when the air conditioning unit engages, turn the car off and tighten this belt again. Repeat this part of the process until the squealing stops.
Check for the serpentine belt diagram under the hood before removing the belt. If it is not there, draw a diagram of this belt so that you can replace it correctly.
Never attempt to work on any of the belts under the hood of a vehicle unless the engine is off.