These instructions will help you setup MFA through the Microsoft Authenticator app if you have not already done so by following the above instructions. It is also the same process to add an additional Mobile device to your MFA options.

1. Make sure you have downloaded The Microsoft Authenticator App to your mobile device
2. Log into the Office 365 portal page : https://portal.office.com
3. Click on your user icon, near the top-right, and select "View Account"
4. Click "Security Info"
5. You should see a list of all your current MFA and password recovery options
   (NOTE: Email is only available for password recovery, not MFA)
6. You can click the "+ Add method" and it will ask you which method you would like to add
   1. Authenticator app - will let you add another mobile device, in the same manner as before
   2. Alternate phone - will let you add another phone number to text you
   3. Office phone - will let you add a phone number to have it call you
      
      
7. Once you have setup the additional option click the "Change" link next to "Default sign-in method" and select the new option you just setup
    
8. You can then click the Delete option to delete the old options you no longer what to use
    
9. You can also click the "Change" link next to a phone option to change the phone number as well, instead of adding and deleting a new one.