If you would like to change your custom domain you can do so from your Site Preferences. It's just as simple as setting up a new one, you can get started by clicking the "Change Domain" button. Once clicked you will be guided through the setup.
URLS from your previous domain
Please note that any URL's shared under your old custom domain will show a "Page Not Found" error if you change your domain. You can remedy this by setting up a forward from your old domain to your new one. You can do