<p>I have a database with user 'dbo' that has a login name "domain\xzy".  How do I change it from "domain\xzy" to "domain\abc".</p>

This is a Windows login, not a SQL Server login, so you cannot 'change' the login name since it is linked to the user account in Active Directory.
Create a new Server Login (Windows) mapped to the new windows user (and remove the old one if necessary). Then in login's Security > User Mapping, permission that login to the appropriate database as user 'dbo' (or assign to the db_owner role)