In order to delete a custom domain mapping from your app, your account must have the App Engine Admin role (roles/appengine.appAdmin) or a custom role that contains the appengine.applications.update permission.
In the Google Cloud console, do the following:
Go to the Custom Domains tab of the App Engine Settings page.
Go to the Custom Domains page
Select the custom domain name and click Delete.
Alternatively, you can use gcloud commands or the Admin API to delete custom domains.
Regards,
Rachel Gomez