Click on the display name of the account you wish to add an alias to
Select the E-mail addresses tab
Click Change (below the primary e-mail address)
Click 'Set as primary e-mail address' next to the e-mail address you wish to set
7. If you wish to change the user's login to the default e-mail address, tick the box marked Synchronize Service User's login, then click Finish. This completes the change of the default e-mail address.