How do I go about changing the AC compressor.

You will have to take it to a mechanic that can remove the refrigeration gas, this must be doe with a machine that collects the gas and do not let it into the atmosphere, change the compressor and receiver dryer using new seals oiled with refregrant oil and then get the system vacced down and re charged with the correct weight of refregrant, this is very important for maximum efficiency of cooling.