Step 1: Login to your Office 365 admin center using the below URL and click on ‘Admin’ link.
Step 2: Go to Admin >> Edit a user
Step 3: Click on the email account name that you wish to change >> Edit username/Email
Step 4: Under Aliases, locate an existing alias account added for the respective email account
Step 5: Now, Click on “Set as Primary” for ’firstname.lastname@example.org’ email account to make it as primary email account.
Step 6: Click save to apply the changes.