You can try going here.
On your computer, go System Preferences/iCloud and switch Keychain off then on.
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