Change number for 2 step verification - where?

I have the same issue. It is because we changed cell phone numbers. The number Apple send 2nd step of verification too is no longer owned by me. I removed the phone and number from my Apple device page but Apple continues to use this old number my cell carrier took back when I upgraded phones. I called Apple Support and explained the situation and spend 45 min with an employee who really could not grasp the issue here. So no Apple keychain for me. Nevertheless, I would like to know how to convince Apple to send 2nd step of verification to current iPhone phone number.

On your device, go to Settings → Your Name → iCloud → Keychain and switch Keychain off then on again.

Tap "Approve with Security Code", tap "Forgot Code" and then tap "Reset".

Follow the instructions to reset iCloud keychain and create a new passcode.

You will then be prompted to update to your new number.

Learned from Zinacef

You would normally make changes at https://appleid.apple.com/ and click Manage Your Apple ID but if you are saying that you cannot do that because it requires verification which is being sent to a number completely unknown to you, then I would contact Contact Apple for help with Apple ID account security - Apple Support

Thank you, Eric. I took your advice and it quickly resolved my problem.