I am trying to change my password but I need to change my phone number

Looks like your Apple ID is Two-Factor Authenticated

Two-factor authentication for Apple ID - Apple Support

To change the password for this type of account, you may need two things:

1. Access to the trusted phone number where the verification codes are sent to - if you've changed phone numbers, you may need to contact your carrier and ask them if they can still provide you a SIM card that has the same number as the old one so you will be able to get the codes. They are sent Via SMS. Once you have access to the trusted phone number, you can then use the Apple Support App on another iPhone to reset the password. The steps are outlined here If you forgot your Apple ID password - Apple Support
2. Access to an Apple device where your Apple ID is currently signed in AKA the trusted device. The steps are outlined here If you forgot your Apple ID password - Apple Support as well

If all else fails, you may want to consider requesting for an account recovery:

How to use account recovery when you can’t reset your Apple ID password - Apple Support

If you have generated a Recovery Key before and lost it, you would need a trusted device that has a passcode lock to create a new one. Account recovery does not work if you have a Recovery key.

Hope that helps!

If you can get a code and sign in, you need to change/add the number on your Apple ID. Look in the Security section when signed in online. You may have to add the new number before you can remove the old number. 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