How do you change your SIM card number?

Each SIM card has a permanent internal code burned into it; cannot be changed save by replacing the chip. The carrier, which knows and is the only entity that can use that code, maps it to a regular phone number that you will be given when contracting their services: voice, data or both.

To change the number, you will have to contact the carrier so they remap the internal SIM code to a new number. There will probably be a service charge and maybe even an explanation as to why.

You mean your actual phone number? The one people can call you to?

You would need to contact your network service provider and ask them to assign you a new number. Its not something you can just unilaterally do yourself.