Reset MFA Or change phone number on Azure AD

For a Global Admin

1. Sign into the Azure portal. - https://portal.azure.com/
2. Browse to Azure Active Directory > Users > All users.
3. Choose the user you wan to reset.
4. Go to authentication.
5. Click Require re-register multifactor authentication

If want to change his/her phone number:

6. At the top of the window, select + Add authentication method.
7. Select a method (phone number or email). Email may be used for self-password reset but not authentication. When adding a phone number, select a phone type and enter phone number with valid format (e.g. +1 4255551234).
8. Select Add.

For a user itself

1. Sign into the https://myaccount.microsoft.com/
2. Go to My Sign-in-> Security Info -> Add sign-in Method