Currently, should a candidate want to cancel or reschedule their tour, they would need to get in touch with you or your admission office directly. What you can then do is one of two options:
1. While speaking with them, adjust their Event for them within their onBoard profile, on their candidate record tab. Here you can edit their already registered tour and then update with the correct information.
2. You can delete their current registration and ask them to re-fill out the form. Before using this option though, be sure to edit your Event Registration form, and on the General Information screen check off the box for "Allow Authenticated Users to register multiple times".