Thank you for reaching out to the Community. That is a great question! Unfortunately once a contact is registered, they are unable to modify any add-on items. I do apologize for the inconvenience. The good news is you are able to cancel their registration, and then your contacts will be able to re-register and select the add-on items. You can click here to follow the steps of cancelling the registrants.
In the meantime, if you have any further questions please feel free to reach out to us here. We are always happy to assist you in any way we can.