I need to change the phone number Appple uses to send verification codes.. How?

Add or remove an SMS number

1. Sign in to your Apple ID account page.
2. In the Security section, click Edit.
3. Click Add a Trusted Phone number.
4. To add a number, enter the phone number and verify it with a text.
5. To remove a number, click next to the phone number you want to remove.

Here's the direct link to the guide 🙂

About two-step verification and SMS - Apple Support