Private facing API requests (i.e. called within server-side requests) should have client_id_key and client_secret_key set in the request headers.
You should never expose the client_secret_key in publicly accessible areas such as GitHub, client-side code, and so forth. You can recycle your keys if you feel you have inadvertently exposed the keys at anytime.
Theclient_id_key and client_secret_key can be recycled in the settings page which is accessible from the navigation bar if you have a paid account. Make sure you update any code that has reference to the old keys to reflect the new keys.