Hi greathornedowl,
Welcome to Apple Support Communities.
The steps here will get that phone number updated for your account: Two-factor authentication for Apple ID - Apple Support
"Update your trusted phone number on your Apple ID account page
- Go to appleid.apple.com.
- Sign in with your Apple ID.
- Go to the Security section and click Edit.
If you want to add a phone number, click Add a Trusted Phone Number and enter the phone number. Choose to verify the number with a text message or automated phone call, and click Continue. To remove a trusted phone number, click next to the phone number you want to remove."
If you can't sign in with one of the two-factor verification methods listed in the resource, you will need to use this method that involves account recovery:
"Update your trusted phone number through account recovery
- Go to appleid.apple.com.
- Sign in with your Apple ID.
- Select “Didn’t get a verification code?”, then select More Options.
- You'll be redirected to iforgot.apple.com.
- Complete your account recovery request. Learn more about account recovery. "
We hope this helps.
Cheers!