update registrant custom fields after initial registration

Use Case:  At time of registration for a camp, we ask for dietary requirements and allergies.  The parent does not complete it at time of registration or wants to update it.  Currently we cannot update this custom field - BUT it is so important that we can.  We use this field to give to our chef for doing camp lunches - now it is obsolete.

Rally answer was:  Collecting it again - is a bit of a nightmare to say the least...

Short answer: you can’t edit event/program registrant custom fields after registration.  Those fields are designed to be completed by the registrant during sign-up. After that, admins can only view the responses by hovering over the “i” icon next to the registrant’s name in the event list. There isn’t a documented way to update or overwrite those values later in the system.  If you need to change the information, the typical approach is to collect it again (for example via a follow-up form or message) rather than editing the original field.