The other day I needed to change the default email address on my Office 365 mailbox. I was hoping it would be as easy as logging in to the Admin Page and selecting the address, but we sync our local Active Directory to Office 365. After searching I found I needed to edit the local attributes of an account in Active Directory Administration Center to change the default email address. The attribute that controls e-mail address assigned to a user is the proxyaddress field. This attribute lists all email addresses that are attached to a user’s mailbox. The address that is listed with the SMTP: before it is the primary address. You can add more addresses by starting it off with smtp:.
Here is a Microsoft Support Article on this topic that is helpful. Step Number 4 is what is described above.