If you want to delete a course you need to:
1. Open the list of your courses.
2. Tap on the icon in the upper right corner of your screen.
3. Tap on the delete icon near the course you want to delete.
4. Tap the button "Delete".
Note that you can only delete the courses that are not active at the moment of deletion.
If you delete a course and then download it again, your progress for this course will be saved and you will be able to start from where you've left off.