If you are a free user, you cannot change your course.
If you are a Pro user, you can enroll in any course. To do this follow the steps below:
1. Go to the 'Courses' tab (the home screen)
2. Tap on your preferred course
3. Tap the 'Enroll in Course' button
You're now enrolled in that course and can start studying it immediately. That course will also be shown on your 'My Courses' tab.