Changing an alternator isn't as difficult as you might think, but it has to be done with care. The first step is to disconnect the negative cable from your car battery. This ensures that you won't have any hot wires going to the alternator. Once done, you can disconnect all the wires from the alternator itself, and you should label them so that you reconnect them properly once the new one is installed.
The next step is the tricky part. You need to locate the tensioner pulley and use a wrench or socket to turn it and release the belt. Then you can remove the belt from the alternator. Different cars have different types of pulley, so check to see what type you have and what size socket will best fit it. Also, be sure to note the routing of the belt so that you can get it back on properly once the new alternator is installed.
Once the belt is out of the way, just remove the bolts that hold the alternator to the bracket and you'll be able to lift it out. Most GM models have three bolts: two in the back and one in front. Now you're halfway there, and the second half should be a bit easier than the first.
Compare the old alternator with the new one to be sure it will fit properly. Check that the bolt holes line up and that the belt will fit the same. Then, start by bolting the new alternator back onto the bracket. Ensure that the bolts are properly tightened and secure.
Once done, it's time to put the belt back on. This is an important step as you need to make sure the belt routing is correct as well as the tensioning. When the belt is properly routed, take your socket and re-tighten the tensioner to the proper setting.
You're almost there! Now all you need to do is reconnect the wires to the new alternator, and then put the negative cable back onto your battery. That's it. Then start your car, and let it run for a bit to be sure the alternator is charging the battery as it should. If you have any problems, take your vehicle to Reliance Auto Centre, an experienced mechanic in Sydney, for further assistance.