This is a behavior in some courses (e.g. SCORM packages) that cannot be changed by the user nor the admin. If you want to review the prior slides before taking the exam, you will need to reach out to support. What they will do is to restart your enrollment; which allows you to take the course again from the beginning. Please note that this doesn't change the course behavior; upon reaching a point in the course (in this case the Exam slide) you still won't be able to navigate backwards even after your enrollment has been restarted.