How do I change the principal name in Active Directory?

Open Active Directory Domains and Trusts. Right-click Active Directory Domains and Trusts in the Tree window pane, and then click Properties. On the UPN Suffixes tab, type the new UPN suffix that you would like to add to the forrest. Click Add, and then click OK.

For More Information Please Refer: https://social.technet.microsoft.com/Forums/windows/en-US/4e4cc3d7-8774-4978-8d52-04a5b5994923/changing-upn-login-name-implications?forum=winservergen