My phone number has changed how do I reset my two step authentication

You need to change the number on your Apple ID. After the change, allow up to 24 hours for the change to sync. Before trying to make the change, sign out of Messages/FaceTime on all devices/computers.

Apple ID - Manage

2FA codes default method for sending is iCloud encrypted push notifications. So as long as you’re still signed into iCloud on that phone you will still get codes just fine.

But, for a backup means of getting codes, login at https://appleid.apple.com/ and edit your trusted telephone number there.