How can I put in a new number on my iPhone 13

If you're asking about telephone service, contact your carrier. They should do it as part of setting up service with them.

If you are doing this remotely, they should send you a SIM card that you can use to replace any existing one. Read this support article for how to do that: Remove or switch the SIM card in your iPhone - Apple Support

Contact your carrier ansd ask a new number.

Then insert the new SIM card in the iPhone.