Not certain on your particular tractor but try these suggestion.
1. Remove ground cable from the battery. This will prevent any electrical sparking or other issues.
2. Take photos of the wiring connected to the starter to make re installing it simple.
3. Remove all electrical connections to the starter
4. Remove the 2 or 3 mounting bolts
5. pull starter straight back until it is free of the engine.
6. take to independent repair shop as this will be the cheapest route