How do I change it?

While there is not a guide on here, the process for replacing the compressor itself is fairly simply. However, replacing it alone is not advised. You also MUST replace the receiver/drier (black canister behind the front right headlight). You should also consider replacing the expansion valve (silver rectangle behind the glove box). I would also suggest a full replacement of all o-rings in the system (since there may be leaks) and then a full flush of the system (since when your old compressor failed it may have sent metal shavings in to the system). You will also NEED to do a full flush if you're switching from R12 to R134a anyway.

That said, the compressor is held in place by three LONG bolts. You start by loosening them all so you can move the compressor in towards the engine block to give slack to the belt so it can be taken off. Then you can take the three bolts out. As you remove them, make careful not (or take pictures) of the placement of spacers for each bolt as they must be put back in the same locations.

The compressor will come out then. You will need to install the new compressor with new seals or o-rings (seals if it's brand new compressor...o-rings if it is an older version that was rebuilt). Before mounting the new compressor fill it with a few ounces of oil, turning it by hand to work the oil in. This coats the internal components with oil to keep it from running dry on first startup.

Then mount the compressor back to the car. Before putting on the belt use a compressor clutch wrench to turn the compressor until it spins easily. This slowly pumps the oil you poured into the compressor back OUT in to the lines which keeps the compressor from hydro-locking from being filled with liquid on startup.

Install the belt over the pulley and tension the belt before torquing the mounting bolts down to spec. Torque specs can be found via Google or in the factory service manual.

The process for installing the drier and the expansion valve should be easy to find online. You can also find general guides and videos online for vacuuming and filling the A/C with refrigerant. Good luck!