As the Admin you can reset Lessons and/or change an individual Lesson Status to Not Complete. Either option will allow the Lesson(s) to still appear for the Learner the next time they open their portal and look at My Courses.
To reset the lesson activity, go to Users and select the name of the user. Then select their User Transcript from the Actions box on the right side of the screen.
Then select the course you want to modify. Scroll down the Edit Activity page until you see the lesson you want to modify. Click the refresh button to reset the status of the lesson.
You can also modify the completion of the lesson without modifying the lesson attempts, by selecting the edit option (pencil icon.) The Lesson Activity box will display and you can then select and save a new status.
This will not modify the overall Course Score or Status.