Registering with a credit is only possible within 48 hours of class time.

Option 1. Go to the weekly courses page and find the "Single-class or Credit" Registration window near the top which displays what classes are available within the next 48 hours. Its default option is to subtract a credit if your account has one:

Option 2. Every course page has a "Single-Class" or "Use a Credit" window depending on if the student has a credit. Click a course page link within 48 hours of class time and find its "Credit Registration" window:

You can cancel the credit registration no later than 1 hour before class time by clicking to expand the  My Registrations ▽  module to get the credit back. Within 1 hour, the credit is lost.