ipad mini - how to change from one apple phone number to another apple phone number

You can't change phone number on your iPad.

Do it on your iPhone that uses the same Apple ID as your iPad.

iPhone's Settings > Phone >

My Number > the number that is active for receiving and making phone call

Calls On Other Devices > make sure the iPad is selected

iPad do not have phone number.

The "phone" number is only for your carrier to bill your cellular data usage.

If you also have an iPhone using the same Apple ID as your iPad, it is then possible to make and receive phone calls