Changes to deadlines and validity lengths are only applied to enrolls made after the changes. To modify the deadlines and validity lengths of currently enrolled learners, navigate to Teaching and Courses. Click on the course and then on the Users & Groups tab. Find the learner that you want to modify deadlines for and click Details.
Once the new page is open, click on Override/Set Dates at the top. From here, you can modify the days when they can access the course, as well as the day that they have to finish the course by. Click Save Changes once done.