There are two options:

1. Use the "Single-class or Credit" Registration window on the weekly courses page whose default option is to subtract a credit.

2. Your  My Registrations ▽  section has a green "Use a Credit" button that displays the same available classes as on the weekly courses page and provides links to those courses where you will find a "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.