1. Log into the Office 365 Portal and click users and groups.
2. On the users and groups page, click the Display Name for the user whose domain name will be changed.
3. On the menu below the user's name, click details.
4. Select the domain drop-down box to choose the domain name to assign to the user and click Save.
5. On the confirmation window, select Yes to confirm the change.
5. For the final window, click Sign out to log out of the 365 Portal. Use the new credentials to sign back in.