With credits, register with the "replacement class" system within 48 hours of class time.

Expand  My Registrations ▽  and click the  Replacement Classes  button for "Available Replacement Classes".

Click or tap the link to a class's course page to register as below.

You can cancel the registration at least 1 hour before class time in  My Registrations ▽  and get the credit back. Within 1 hour, the credit is lost.

Also see:

How to cancel a class online to receive a credit