How can I change my phone number as a recovery to received a “code”, currently I have an old number .. I’m on an iPad,, I can’t find the settings,,this is ridiculous,

I can’t find the settings to change my old cell phone,, I have a new one and I need to change just in case I need to receive a code. I’m on iPad and I can’t find this settings. The profile settings doesn’t have a form to change this information. Thanks I need so e help