You'll have to re-read and put each word individually back to unknown. Unfortunately I don't think it's possible to do all at once for a whole lesson.

The quickest way to do that is probably by going to Lessons -> Vocabulary (it's on top left of the page) and setting the filter to Known Words and sort by Creation Date, and choose the specific lesson if you want.

That will list only the known words and sorted by the most recently added ones. From there you can select all the words you want and click More Actions -> Remove, which will reset their status, or you can just make them into LingQs ;) Unfortunately there's no way to just reverse progress of a specific lesson as your words and LingQs are saved on the account, so to say, rather than bound to lessons.