By default, when your registrants submit their registration they have the ability to return at a later time and edit fields that are not associated with an additional charge. This means that attendees will have the ability to change their response to custom questions and personal information. If a registrant would like to edit their registration, there is a link on the confirmation page and the confirmation email automatically sent to the registrant.
If you do not want your attendees to be able to edit their registration go to the "Registration" section of your event. Select "Settings" in the upper right corner. Under the section header "Registration Options" there is an option "Disable Edit Registration for Attendees".
Checking this box will disable the attendees' ability to edit their registration on both the confirmation page and confirmation email. Unchecking this box will allow your attendees to edit their registration. Don't forget to "Submit"!