Event organizers set their own cancellation policies. Generally speaking, paid tickets cannot be cancelled. It may be possible to cancel FREE tickets if the organizer allows. You can determine if your ticket can be cancelled by accessing your ticket order from your account. If the organizer allows cancellations, there will be a "Cancel Order" link below the Print Tickets button.
If you need further assistance, you should contact the organizer directly. You can contact the organizer from the organizer's profile page.