I need to change my phone number

To change the number, you would need to start a recovery. Unless you are signed in with that Apple ID on another device where you could then change the phone number in settings. The following directions are assuming you know the Apple ID password and you know the old number and have access to the email.

Go to iCloud.com, sign in with that Apple ID and password, when it asks for the code sent to the number you no longer have, click did not receive verification code. Then click more options. It will take you to iforgot.apple.com. You will be asked to enter the old number. After you enter the old number click continue. The next page click the third option, “Don’t have access to any of your Apple devices or your phone number?” On the next page, click continue anyway. The next page will ask for a code sent to that email. Enter that code. Then the next page will ask you for a new number. Enter the new number, you will get a code either by text or phone call, depending how you chose to receive the code. After you enter that code, it will give you instructions on wait time. But because you were able to enter the password, you might not have a long wait time. It takes 24 hours for the system to examine your account to see if you are you. Then in 24 hours, go to iforgot.apple.com, enter the old number, and it will tell you how long you have to wait. Could be 1 day, 2 days or a week…what ever it says is what you have to wait. At the end of your wait time, it will contact that new number to tell you that you can now use the new number. During your wait time, you can always go back to iforgot.apple.com and check your progress. Remember to need to always enter the old number in iforgot.apple.com to check your progress.

the reason I said to go to iCloud and try to Sign in with Apple ID and password is because then the system knows you know the password. It could lessen your wait time.