If you need to cancel a session you have booked, the easiest way is to go through the email confirmation you will have received when booking.
This will have a 'View and manage' link somewhere near the bottom that will take you to the EventBrite website.
You may need to create a password if you have not logged in to EventBrite before, but it should know what you have booked as your tickets should be linked to your email address.
It should take you straight to your ticket and there should be a 'Cancel Order' button so you can cancel it. The site looks very similar when browsing on a mobile.
If you do not get to this page, you should be able to get to your tickets summary by clicking on the 'Tickets' link in the top right of the EventBrite page once you have logged in or there is also a 'Tickets' link in the user menu (click on the person symbol in the top right.) Once you reach your summary page, click the ticket you would like to cancel and it should bring you to a page with a 'Cancel Order' button like the one above.