If you change your mind about attending an event, you can unregister yourself from the session by first going to the Events tool (found under the Communication suite), and clicking on the name of the event.
If this event's registration mode is set to Self/User or Either, you will see an Unregister button for the session that you are registered for. Simply click this button, and you will be prompted as to whether or not you would like to unregister from the session. Click OK, and you will no longer be registered for the session.