Powershell script to bulk change user display name using CSV file

There are a few script which you could use. this is one for example => https://mymicrosoftexchange.wordpress.com/2015/11/20/office-365-script-to-bulk-change-the-userprincipalname-to-match-the-email-address/

Are your users sync'ed or cloud only users?

If they are sync'ed, you need to do this in AD.