To install a pressure switch for an air conditioner, first safely and legally recover the refrigerant used in the air conditioner. Consult your vehicle's service manual to find the location of the pressure switch. Unscrew the old switch and replace it with the new one, including a new rubber seal.
After installing the new switch, use an air conditioning gauge and vacuum to pump all moisture from the air conditioner. Attach the gauge to the air conditioner service ports, and then attach the vacuum pump to the gauge's service hose. Run the pump for an hour after opening the gauge's service valves, and then close the service valves and remove the vacuum. Leave the gauge set attached, but attach the can tap to allow for refrigerant replacement.
Secure the refrigerant can tube to the can tap, open the can, and open the blue service valve on the gauge. Use pressure from the can to fill a portion of the air conditioning system before turning on the engine. Turn the air conditioner to its highest setting and allow the can to empty into the system. Use as many cans as needed to your system's capacity, as listed in the service manual, closing the service valve each time you secure a new can to the gauge. Close all valves on the gauge, turn off the engine, and remove the gauge.