This is a very simple job, although getting to the screws is a little tricky. The alternator brushes and regulator come as an assembly. There are no wire connections to the assembly. Two phillips head screws hold the assembly in place. The rear screw is easy to get to with a standard #2 phillips screwdriver. Access to the front one is more difficult, as the oil dipstick tube, knock sensor and various wiring harnesses get in the way. I found it helpful to remove the two 10 mm bolts that secure the top of the dipstick tube. I then removed the upper bracket and the dipstick/cap assembly. I DO NOT RECOMMEND TRYING TO REMOVE THE DIPSTICK TUBE, as they get brittle with age (I don't even know whether it is a press fit or a screw fit into the block - I just left it alone). Removing the upper bracket allows the tube to be bent fore and aft enough to allow a bit better vision and access to the screws.
I replaced mine at 118 thousand miles. There was about 1/8" left on one brush and about 3/16" left on the other, New brushes are between 1/2" and 5/8" long, so mine were getting pretty close to the end of their life. Another consideration is that when they get that short, the brush spring tension probably isn't as high as it should be.
The brush/.regulator assembly is at the bottom of the alternator. Look at it from the opposite side of the car to get a better view.