how to replace the air cond. pump

Have a mechanic handle this. You will likely not have the tools to recapture and recycle the refrigerant (releasing refrigerant into the atmosphere on purpose is around a $5,000 fine), and you may not be able to recharge the system properly. Most likely the A/C compressor clutch bearings are either gone, or going, and it has either locked up, or is causing the belt to slip because it is too hard to operate. Expect to pay around $500 for the job at a local mechanic in most areas, including parts and a recharge.

Can you replace the pulley on the a/c compressor