how do i change the default phone number on my iTunes account?

You can change the phone number on your account by logging into it via the Store > View Account menu option on your computer's iTunes and to the right of your billing address clicking the Edit button

Or you can edit the phone numbers on your account by the 'manage your apple id' button on http://appleid.apple.com

That works only for the account contact info. The phone number I want to changes is the "reachable at" phone number

Are you talking about the phone number for iMessages/Facetime? If so, it can only be an iPhone number. Sign out of iMessage/Facetime on all of your devices. Then, sign in on the iPhone whose number you want to use. Once it's activated, sign in, using the same Apple ID on all of the other devices.