Change primary email address in Office 365

Start powershell as administrator

Run the following commands:
$LiveCred = Get-Credential
(Enter your Office 365 administrator credentials)
$Session = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri -Credential $LiveCred -Authentication Basic -AllowRedirection
Import-PSSession $Session

Find all aliases for one account:
get-mailbox | select -expand emailaddresses alias
get-mailbox | select -expand emailaddresses alias

Add email alias to an Account:
Set-Mailbox -EmailAddresses,,

NOTE: the first address after “SMTP:” will be the primary e-mail address. All aliases will be replaced. If you have 100 aliases and only type in one in this command, you will lose 99 aliases!