Step 1: Login to your Office 365 admin center using the below URL and click on ‘Admin’ link.
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https://portal.office.com/
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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 ’email@yourdomainname.com’ email account to make it as primary email account.
Step 6: Click save to apply the changes.