@IBN Firstly, there is no point in @ mentioning lots of people, this is a community forum and anyone might offer great advice.
I assume what you are seeing is the users UPN, what you need to do is change the UPN. This needs to be done with great care, and the process will differ based on exactly how your users are created in Office 365. If you are syncing from a local AD then you would start there, see Prepare for directory synchronization to Microsoft 365 - Microsoft 365 Enterprise | Microsoft Docs
If your users are created in the Cloud, then check you have your preferred domain registered fully in the domains section of M365 admin, then you can go to each user and edit their "Primary Username and Email" to the preferred format.
