It is easy to change your attendance if you have the app. If you enter the event and press the button that says “Attending” or “Decline” the opposite alternative will appear and you can simply choose another answer.
To see past or declined events, press "upcoming" in the event overview to change the filter. You can also chose if you would like to show repeating and declined events in the overview.
Email: Those who reply through email notifications can change their answer by using the same link they received the first time they were invited to the event. If you register and log in on www.spond.com you will be able to get a full overview over your events.