factory is 14 tooth, you want to go with 13 tooth if you are going to go down one.
easy to do.....
1) take cover off (two bolts)
2) take two bolts off the sham
3) take ring off
4) slide out the old gear
5) take old gear out
6) put new gear in
7) bolt it back the same way it came off
8) and you are done with the install.....
then comes the fun part, adjusting the swingarm to reposition the chain.
9) first, take off the brake caliper... two bolts
10) there are four bolts on top, 12mm and 17mm. loosen them.
11) there are two bolts on the end plate, loosen them as well....
12) move the axle by hand to adjust accordingly.
what you are looking for here, on top of swingarm, is the lines on the lower two bolts match up. This will tell you are properly aligned. Aside all this, you adjust the axle in a position to allow approximate 1.0-1.5" of chain slack. You achieve this by loosening and tightening the bottom two bolts. Everytime you make an adjustment, wiggle the swingarm to get an accurate reading on where your axle is at. It will not slide on its own. Once you have achieved this, then you tighten the four main bolts, and then tighten the two end bolts. and wha-la, you are done.....