There are two options:

1. On the weekly courses page, you'll find the "Single-class or Credit" Registration window near the top. Its default option is to subtract a credit if there is one:

2. Every course page has a "Single-Class" or "Use a Credit" window depending on if the student has a credit. By tapping to expand your  My Registrations ▽  section under the top image of a mobile device (or right sidebar on a desktop computer), you'll find a green "Use a Credit" button that opens a page with the same available classes as on the weekly courses page (above) and provides links to those courses where you will find the following "Credit Registration" window:

You can cancel the credit registration no later than 1 hour before class time in  My Registrations ▽  and get the credit back. Within 1 hour, the credit is lost.