See -> Two-step verification for Apple ID
Two-step verification for Apple ID – Apple Support (UK)
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Where should I keep my Recovery Key?
Keep your Recovery Key in a secure place in your home, office or other location. Consider printing more than one copy, so that you can keep your key in more than one place. Your key is easier to find if you ever need it, and you have a spare copy if one is ever lost or destroyed. You shouldn't store your Recovery Key on your device or computer, because that could give an unauthorised user instant access to your key.
If you ever need a new Recovery Key, find out how to create a new Recovery Key.
What if I lose my Recovery Key?
You can sign in to your Apple ID account page and create a new Recovery Key using your Apple ID password and one of your trusted devices.
- Go to appleid.apple.com/uk.
- Sign in with your password and trusted device.
- In the Sign-In and Security section, click Recovery Key > Change Recovery Key.
Keep your new Recovery Key in a safe place. Your old Recovery Key won't work any more and you can't use it to reset your password or access your account. If you can't see Recovery Key in the Sign-In and Security section, then you are not using two-step verification.