Hi
My name is Daniel. I am an Independent Advisor.
You can replace the old verification method either by removing it on your own or ask your admin to reset it.
Since you use an institutional account (work/school) please reach out to your admin & ask your admin to disable the 2FA authentication on our account for a while: https://docs.microsoft.com/en-us/answers/questions/101179/how-to-disable-the-two-factor-authentication-from.html
Or you can ask the admin to reset the 2FA authentication method to use MS Authenticator app instead of using a mobile number because as per my personal experience cellular networks sometimes have trouble delivering SMS: https://answers.microsoft.com/en-us/msoffice/forum/msoffice_o365admin-mso_password-mso_o365b/reset-2fa-for-office-365-account/caec62dc-3603-4a3d-9670-01b1ddc3e519
After you have successfully regained access to your account & you want to replace the verification method on your own then you can visit this link: https://mysignins.microsoft.com/security-info
Disclaimer:
This is a peer-to-peer support forum. I'm an Independent Advisor & fellow MS forum users just like you, who are trying to help other users with their issues. I'm Non-Microsoft Staff.
Kind Regards