1. Uninstall the app,
2. Delete folders:
a) C:\users\username\AppData\Roaming\Yousician Launcher\Yousician.app\
b) C:\users\username\AppData\LocalLow\Yousician (the folder directory may vary depending on your Windows username.)
AppData folder might be hidden so you might need to do this:
3. Run a Disk Cleanup to remove temp files, and
4. Reinstall the app.
If you are running Yousician under guest or standard account give this account Admin powers (for installation only) so YS could place all files in the right folders (one of Windows updates made some sys mess) .