It isn't possible to restart the Plan but you can change your goals, difficulty level or schedule at any time in the Plan Settings.
If you'd like to start from scratch, you'll need to reset your progress - though this will remove all your workout history and achievements.
To do this:
1. Open Settings in Seven and tap Edit Profile
2. Tap the 3 dots in the top right corner of the screen
3. Choose 'Reset Workouts'