If you need to remove RSVPs from an event, navigate to the event and look for See All RSVPs. Click this option to view the RSVP pop-up.
You should see a wrench icon next to each RSVP. Hover over this icon and click Remove. This will remove the RSVP from the event and it will re-open the ability to register if the event was at full capacity.
Follow these same steps if you'd like to remove your own RSVP to an event.