In order to change your Display Name in Microsoft 365 email you need to first add or modify your preferred name in your system of record.  The system of record for Faculty and Staff is Workday and the system of record for Students is the Student Information System.  Microsoft 365 email will reflect your new Display Name one day after you have made the change in your system of record.

Note: Mobile devices (e.g. iPhone, etc) will also update automatically to display the name designated as preferred name or default name. There may be a delay before the device synchronizes with the systems of record.

For additional information, see Email Addresses at Yale - FAQ.