If you need to update your o365 MFA phone number, that can be done via self-service on the www.office.com portal. Follow the below instructions to get that done.
NOTE: These steps can only be performed if you can login to the office portal without issues. If you cannot login, you may need to open an IT ticket via the support portal to have your MFA reset completely.
1. Login to Office 365 via www.office.com
2. Click your initials in the upper right hand corner and hit 'View account'
3. On the next page under Security info, hit 'Update Info'
4. From this screen, you can add a sign in method or change the phone number from your old one to your new one using the change option. You can also delete the current sign in method and add a new one if you no longer have access to the current one.
After your phone number is saved, you should be able to to verify with the new number/authentication method.