You’re not missing anything, as that’s not a current feature. If it’s not in the documentation, then it’s not a feature:
https://stephensherrardplugins.com/docs/pta-volunteer-sign-up-sheets-documentation/
If a user has made a mistake, and you have it set for them to be logged in to a user account, then they can clear their existing signup and simply sign up for the same thing again with the correct info.