A phone number that is listed for a contact cannot be changed. The contact will need to opt-in with their new number. Once they do, the new number can start to receive text messages. 

If there are loyalty check-ins associated with the old number, then once the new number is added, the Kiosk Check-In Manager Feature can be used to update the check-ins. 

It is strongly advised to go into the mobile campaign Advanced Settings and block all messages for the campaign before adjusting the check-ins so that the contact does not receive any rewards messages from the system. Then use the check-in manager to redeem or remove those offers that they have already used.