1. Go to office.com
2. On the bottom left, press on your user account
3. Click on View account
4. Then under the Security info press on the Update Info link.
5. Click on Add Sign-in Method
6. Choose the Microsoft Authenticator
7. Make sure Microsoft Authenticator app is installed on device, if already installed skip.
- Go onto the authenticator app on phone
- On the authenticator app press on add an account and select work or school
- Then press on Scan QR Code
8. Go back to the office.com website and press next until you see the QR code you need to scan.
9. Then Delete the old phone you see attached to your account.
refer to this article for more help with MFA : Multifactor Authentication (MFA) - Field Instructions