Since the learner has already completed the course, any new material that you add will not be required on their part. To make them go through any new material that you added, you simply need to change their Course Progress from Completed to In Progress.
To change the course progress of a learner, simply navigate to the course that the learner is enrolled in and click on the Learners tab. From there, click on the Course Progress button beside the learner.
All you need to do from here is use the Drop-Down Menu and change their Course Status to In Progress.