It's not currently possible to edit a custom domain name through the app. What you'll need to do is first cancel your subscription through iTunes here: https://support.apple.com/en-us/HT202039.
After you cancel your subscription, create a new site and re-subscribe with the correct domain name. Let me know if you need any additional help!
If you want to use your old site with the new domain, please send us a message through the app or via email with the old domain name and the new one you would like to connect the site to.