From what I understand and personally experienced on the site, individual lessons cannot be "unwatched". I have also noticed that marking a tutorial "incomplete" does not stick and always reverts back to "complete". This is frustrating for many of the same reasons already mentioned in this thread.

For me, I did a search for "finger exercises" and found "Spider Legs Warm-Ups: Great Finger Exercises!" I browsed all the lessons to complete at a later time and now they are all marked "watched" and the tutorial is marked "complete". Marking the tutorial "incomplete" does not stick and always reverts back to "complete" with all lessons watched.