Guests can change their existing submissions in several ways for any options:
Following the 'Edit Reply' link in the e-mail confirmation that was sent after a guest has registered for the event.
From the confirmation screen shown after successful registration and clicking 'Edit Reply'
Using the same e-mail address as used for their original registration and clicking 'Edit Existing Submission' in the next step. An edit link will be sent to their email address and they'll be able to edit the submission after following the link.
Note: Submissions made in DRAFT can not be edited as well as the submissions that have payments associated with them.