When updating the Debian based system, apt-get may display an error message like:
W: GPG error: http://download.webmin.com sarge Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY D97A3AE911F63C51
Type the following commands:
gpg --keyserver pgpkeys.mit.edu --recv-key D97A3AE911F63C51 gpg -a --export D97A3AE911F63C51 | sudo apt-key add -web