Then you need to call customer service or go in to a store to do that change. If it's the primary # in your family share plan I would suggest waiting until the start of your billing cycle if you can. Any change done to that primary # will impact your allowed minutes for the month.
If you call in to CS then make sure you're calling from a separate line so they can help you reprogram the phone. And you may want to back up or forward any voicemails you need because changing a number deletes your voicemail box altogether.
Also, I would highly suggest removing your phone number from your profile information. There may be people on here with nefarious intentions.
Probably why he needs a phone number change in the first place! 
But yes, call Customer Service. As long as there is a number available in that area code, they should beable to help you. Although, the last 4 will be random and they can't do much about that.
Start a new topic or ask questions in the open forum.
