Hi there!
Thanks for getting in touch! We don’t currently have support for cancelling RSVPs built into Event Tickets. Unless you’re using Event Tickets Plus with a payment processor for paid tickets (where cancellations/refunds can be processed through an ecommerce platform), attendees will need to reach out to you directly in order to cancel an existing ticket order. I’m sorry to disappoint!
That said, you may wish to look into our Eventbrite Tickets plugin if this is an essential feature for your project–Eventbrite has similar support for RSVPs/free tickets and does support cancelling registrations for users who are unable to attend.
I hope that helps! Thanks, and let us know if you should have any other questions!