If you have credit on your account, you can apply it to purchase a course.
Click on the course where you would like to apply the credit. You may need to look in the Enroll in a new course section.
Find your new course
If you have not yet entered the course, you will see the payment page. Check the box for Use my credit, then click the button on the right to Use Credit and Enter Course.
Pay for course
If you have entered the course before, such as on a Grace Period, you may need to first click pay for this course and optional eTextbook now? in the yellow banner near the top of the page.