You can reset the last lesson in the stage you're on.
In your dashboard, go into the profile for the learner whose lesson you want to reset.
Here you'll find a double arrow button which you can click to reset the last lesson in your current stage.
Resetting the last lesson gives your learner a chance to start over again with the lesson they're doing. Just select to reset and it will come up again for your learner to do from the start.