Visit your Dashboard page. In the Events window is a pulldown menu listing all events that you are registered for. Select the event and use the "Unregister" button.
It is highly recommended that you use this function if you know ahead of time that you will not be able to attend the event. Unregistering for an event allows another person to take your place in the event. Repeatedly not showing up for events you have registered for will result in a loss of sign-up priority for future events.
This unregister ability is removed 60 minutes prior to the scheduled qualifying.