Mac

1. Remove Syntorial from the Applications folder
2. In Finder, from the Go menu at the top of your screen, select "Go to folder..."
3. Copy and paste in: ~/Library/Application Support/Syntorial
4. Delete that Syntorial folder
5. Go back to "Go to folder..." and copy and paste in: /Library/Application Support/Syntorial
6. Delete that Syntorial folder as well

Windows

1. From the search option in the taskbar, search "Add or remove programs" and open it
2. In "Search this list" type Syntorial
3. Click "Syntorial" in the results
4. Click "Uninstall"
5. Follow the prompts
6. Once it's finished, go to: C:\Program Files (x86)\Syntorial
7. Delete that whole Syntorial folder

Also

Your progress file(s) is stored in a separate location. By default, they're in Documents/Syntorial, unless you stored them somewhere else. If you think you may want to resume at a later date, leave these files there. They're very small so won't take up much space. Otherwise, you can delete this Syntorial folder as well.