Once registered for a course, you can unregister for individual sessions within the course if need be.
Head to the course you are registered onto. In this example, we're registered onto Intermediate Pilates.
Now click on the group you're registered onto. Then click to manage sessions.
You'll now see a list of all the sessions you're registered onto.
Simply un-tick the session you can no longer attend, and save.
You'll now see that you are no longer marked as attending for that session.
You'll always be entitled to the number of sessions you have paid for, which in this case is 9.
If at any point you decide you can attend the session again, just click on the session and add it back to your course booking.