On the SharePoint admin center page , select the check box next to your public website, and then click Website Domains . ...

1. On the SharePoint admin center page, select the check box next to your public website, and then click Website Domains.
2. In the Rename your website dialog box, in the New URL drop-down list, select the domain name you just added. For example, www.fourthcoffee.com.
3. Click OK.