Please am the administrator for my company Office365 account.
Please we have acquired a new domain which I have setup and verified in the admin page.
How can I change the current users email to use the new domain , also after the change will it affect new email coming to the users, finally will the login require the new email address or the old email to login
The easiest way to do this is with PowerShell. You'd find all the user accounts and then update the email addresses assigned to their mailboxes so that the new primary SMTP address belongs to the new domain. Keep the address for the older domain to make sure that email sent to those addresses continue to be delivered. The new primary SMTP address will be used for all new messages.
If you want to change the user principal name (sign-in address for accounts), that needs a separate command to update the Azure AD accounts. You don't have to do this, but it's usually best to have the user principal name match the primary SMTP address.