Hi @AusPhil l,
As far as I know, there are two ways to change email address.
1.In admin center home page, select the User Profile, find the Contact info and edit the Email address directly, as show as below:
2.In M365 admin center, go to the Users > Active users page. Select the user and then on the Account tab select Manage username and email. As below:
Reference: Change a user name and email address - Microsoft 365 admin | Microsoft Learn
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.