Though a video may be marked as watched, you may still click on it to re-watch it.
Just go to the instructor's "Lesson Plan" page you want to review while logged into your account to see the complete list of lessons.
You can find your class's Lesson Plan by signing into your account and clicking the instructor's name from the Library page.
If you're already viewing a video, then you can click on your instructor's name at the top right corner of the page to be brought to the complete lesson plan.