We'd be happy to help you with resetting your lessons! Head over to your Account/Profile settings, located in the far right of your navigation bar and click your "Account Settings." You will then see "Study Data" located on the right side of your profile.
You will see two options to reset your Answer Data and Answer Lessons Data. Resetting your Answer Data will include refreshing all Questions, Answers, and Stats you've input, including notes. Resetting your lesson video data will consist of your historical record of all videos watched.
This isn't something you can undo, so if you decide that you want the green check marks back, you can manually re-mark each video lesson as "I Understand this Lesson". That will bring about the green check marks again!
One important note - there is no way to reset the tasks on your study schedule all at once. To do this, you'll need to uncheck each individual task. I'm sorry about that!
Let us know if you have any other questions, and hope this helps! Happy studying!