How to change trusted phone number for apple ID?

My son is trying to access his icloud email. He is unable to log in because the verification code is being sent to an old mobile phone number which he doesn't have access to. How do we change the trusted phone number associated with his icloud account? We have tried but we are getting a message that the waiting period is 13 days! Is there another way? He has tried it on all our computers and his phone (which is android not iphone).