Keep your Recovery Key in a secure place in your home, office, or other location. You should consider printing more than one copy, so that you can keep your key in more than one place. Your key will be easier to find if you ever need it, and you'll 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 unauthorized user instant access to your key.
If you ever need a new Recovery Key, you can create one from your Apple ID account page. Just sign in with your password and trusted device, then go to the Security section and click Replace Lost Key.
After you create a new key, your old Recovery Key won't work.