To restart a user instance on JupyterHub, follow these steps:

1. Go to the JupyterHub homepage.
2. Go to the admin panel
3. Locate the user instance that needs to be restarted.
4. Click on the stop server option near the username.
5. Once the instance is stopped, look for a start or launch option or button to start the user instance again.
6. After following these steps, the user instance should be restarted and ready to use again.