It can always occur a training needs to be cancelled. When opening the tab 'Upcoming' in the app you can view all upcoming and scheduled trainings.

 
In case you have purchased a training but it still needs to be scheduled, you can locate them in the 'Plan' tab.

In order to cancel a training you would always need to get in touch with the trainer. As only the trainer can cancel your participation.

How to contact the trainer?

Select your Training you would like to cancel > Contact Venue > And a new page will open with the contact details.