Retaking a course you have previously taken is not possible unless the course has been updated. You can check if the course has been updated by looking at the course number and comparing it to the course number in your account.
The course number can be found at the bottom of the Course Overview page & you can find it on your My Courses page (example provided below).
If you would like to retake a course that has not been updated, you do have the option to create another account with a different email address.