Change the user name, not the email configuration.
Change a user's email address
You must be a global admin to complete these steps.
In the admin center, go to the Users > Active users page.
Select the user's name, and then on the Account tab select Manage username.
In the first box, type the first part of the new email address. If you added your own domain to Microsoft 365, choose the domain for the new email alias by using the drop-down list. Learn how to add a domain.
Select Save changes.
Reference:
https://docs.microsoft.com/en-us/microsoft-365/admin/add-users/change-a-user-name-and-email-address