There are two methods for changing your security code. The first assumes that you're using a Mac that is already set up to use iCloud Keychain. This is the preferred method for changing the security code.
It asked my iCloud security code and after it asks for 6 digit verification code that has been sent to my old phone number that I no longer have access to. How can I set iCloud Keychain in case if I changed a phone number?
You can also change the iCloud Security Code or the phone number used to verify your identity after you use the iCloud Security Code. Choose Apple menu > System Preferences, then click iCloud. Make sure Keychain is on and your Mac is approved.
Добавление функции «Связка ключей iCloud», предназначенной для ... iCloud-keychain .... Судя по change list изменений дофига, но работает вроде так же, .... после обновления, включенная в 7.0.2 функция “find my ipad” ..... ключей iCloud» на iPhone нужно создать iCloud Security Code...
The iCloud Security Code is used to authorize additional devices to use your iCloud Keychain. It's also used to verify your identity so that you can perform other iCloud Keychain actions, such as recovering your iCloud Keychain if you lose all of your devices.