If you've completed an entire lesson theme, a checkmark will appear over it.

If you've only completed part of a theme, you'll see a percentage indicating your progress instead.

Click on 'Settings' in the lower left of any page on Chess.com.

Select 'Gameplay' from the menu on the left.

On the Gameplay settings page, scroll down to the bottom to find the 'Lessons History' section, and click the 'Reset' button to undo your progress for lessons.