How do you replace the oil in the air conditioner compressor?

Oil is the lubricant , R-134A is the REFRIGERANT , that causes the 'cooling'. Many possible reasons for not 'blowing cool' , low refrigerant is only one of them. Better to look on youtube for an instructional video , that may be much better help than here. My personal suggestion , after seeing comments by others , showing how much time and money they've wasted on incorrect servicing or just flat-out inability to properly diagnose the system ,is to GET REFERENCES to a reputable shop , and let them check it for you. That's just my opinion , from watching only the stuff on repairpal , and the thousands wasted on A/C , trying to 'save money' (of which I am definitely NOT against!!)... Good luck.