iCloud keychain authorisation sends sms to old number

1. On your iPhone, go to Settings → iCloud → Keychain and switch Keychain off then on again.

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

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

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

on your iphone - Setting > iCloud > Keychain

• Approve with security code

• on the passcode step, click 'forgot code' and then click 'reset'

follow on screen step, you will be able to update your phone number. hope these help

I ended up calling their support on the phone and a staff member changed it for me. Easy solution if you can get through the customer queue