You can allow them to edit their answers after form submission by one of these options:
1. Log in to look up their record (recommended)
Both the form and the portal extensions, allow users to look up their existing records. Ask the users to log in. It will take them directly to their record to edit the info they submitted. You would need to enable the login option.
2. Confirmation email
You can also generate an editing URL for each record. This is ideal if you'd like to email users a link that takes them directly to their records. Send them an auto-response email that contains the form link to their specific record.