If you are getting an error during and ubuntu updage / upgrade command like:
W: GPG error: https://dev2day.de jessie InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 97BC9498DC9666A8
The all you need to add the key to your local instance of ubuntu from ubuntu key servers. The required ID of the key has been defined at the end of the error.
But if the key is not in the database you need to find the path of the key from the owner like plex media.
From Key Server: apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 97BC9498DC9666A8 Manually Add the key: wget -O - https://dev2day.de/pms/dev2day-pms.gpg.key | sudo apt-key add -