If you no longer wish to continue with the domain, you may delete it. Deleting a custom domain should be rare and it does not affect the site contents which will still remain in the sites admin domain.
To delete an existing custom domain, click on 'REMOVE' on the site dashboard.
You'll then be taken to another page to confirm the removal.
Here, you need to enter the custom domain name to confirm deletion. The custom domain is displayed underneath the 'Remove Custom Domain' title, enter it exactly as shown.
IMPORTANT: As noted in the instructions for 'Adding a custom domain', there is a rate limit for the number of times a given domain may be added and removed.
Once you have confirmed that you wish to remove the custom domain, you will be redirected to the site dashboard. Your site will still be accessible via the admin domain but you will no longer be able to reference it with a different name.