For starters, I don't know exactly if you're starter is the same as mine, but i think they are all about the same anyways haha.
1. Disconnect hot cable from the battery
2. Disconnect the red (hot) wire from the starter
3. Unscrew the two small screws from the plastic cover over the starter gear
4. Unbolt the two large bolts holding the starter on (The starter should just lift off from here)
5. If you're starter is like mine, it probably has a flat area where the big bolt holes are at where you can clamp it down. Get two C-clamps and clamp it down to you're workbench. (This is optional, but made it much easier for me)
6. If you have a pin punch, this will be much easier, and i suggest using one so you don't break or bend the pin holding the gear on. (They're pretty cheap at any decent hardware store) Line the punch up with the pin and tap it gently. You may have to hit it harder or use Open & Shut to loosen it. This may take a little work but it shouldn't be too hard. Just be prepared to find it after it flies out.
7. Take the metal guard and the gear off and slip the new one on.
8. After putting everything back on, Insert the roll pin in the hole and tap it back in. It should be centered in between the two sides.
9. Reconnect everything back in the opposite order you took it off and you should be done!
Here's a pic to show what i meant.