Steps which need to be followed in order to update FB webhook are below:
Resource: developers.facebook.com (user login credentials)
1. In the Dashboard, go to Whitelabel >> Services >> Facebook to find the webhook URL and Verification Token.
2. Go to https://developers.facebook.
3. Once you enter into the respective app's Dashboard, click on the Webhooks section in the left hand side menu.
4. On the Webhooks page, click on the Edit Subscription button that will open up a dialog box where Callback URL and Verification Token, as displayed in the Punchh Dashboard, needs to be entered. Hit Verify and Save, once done.
5. As the final step, for each of the fields email, first_name and last_name pick v3.1 from the drop-down and hit Subscribe.
This needs to be done for all the apps in order to let the web-hooks come into effect to update guests' email when they changed it on FB.