As part of the registration process a phone number is required, and is utilized for 2FA for any latter login. This phone number is provided by the user as part of the registration flow in the portal.
If the phone number of a user needs to be changed there are 2 options:
Another admin user with enough permissions can delete the user from all accounts it is assigned to and create it again. As part of the new registration the phone number is requested again.
If deleting the user is not feasible, then you can contact support and request the change. You will find the support contact details in the top right menu of the portal.