You will be able to change your registered phone number(s) within online account access, but there are currently no settings that allow this to be done within Mobile Apps. However, during first-time use of multi-factor authentication, you will be able to change the phone number(s) you use for authentication. This will allow a Mobile or Apps-only user to add a new number or change an old number that was set up in USP, so the user can log in from the registered phone.