You can mark classes as complete without having to individually move through lessons.
How to complete days:
Open any day
Click the 3 dots (...) in the upper right corner
Click "Mark day as complete"
Click the 3 dots (...) again to "Mark days as not complete".
How to complete grades:
Open the quiz for any grade
Pass the quiz
Click the 3 dots (...) again to "Uncomplete grade".
How to complete classes:
Open the final quiz for any class
Pass the final quiz
Click the 3 dots (...) again to "Uncomplete class".
Alternatively, you can go to My classes > Completed, and click the 3 dots (...) in the lesson tile to "Uncomplete class".