If a course is available in the audit track, on the course introduction page will mention a FREE option. Click the Enroll button to enroll.
After you click Enroll you'll have the option to Upgrade Now or Continue. If you select Continue you can enroll now for free and optionally upgrade later if the upgrade deadline hasn't passed.
If you're interested in auditing one of our programs, please read this article for steps to enroll.
You can read more about the free audit track here: What does audit mean? What is the audit track?
Some edX courses, such as Boot Camps and Executive Education courses do not offer an audit track.