If you uploaded your Contacts with a CSV, or manually added them to AskNicely: The only way to update your Contacts is to upload a corrected CSV again to update the users and their fields. This CSV must at least contain a column for "Email."
If you currently have an active integration with your CRM / POS system or an API connection: The Contact needs to be updated within the CRM / POS first, which can then be re-synced over to AskNicely.
If the Contact were initially brought over via an Integration but are no longer bring brought over: re-add them to the list in your CRM / POS system (this will update their information in AskNicely). You can also use a CSV to manually update contact data: to do this, you will need a CSV with an "email" address column.
If the Contact has unsubscribed: You actually cannot update these contacts - there's no need to update the person's segment or any info on the Contacts page because they won't be surveyed by AskNicely again.