Step 1: Go to your account settings.
Step 2. Go to email domain settings and click the button Delete Domain.
Step 4. Once the domain has been deleted, you will have to scroll up and include your new domain.
Step 5. Update the DNS records for your new domain.
Remember to configure and verify the values of your DNS and MX records after adding your new domain. If you want to know more about how to configure them, you can read this article.