If you receive this error from Software Updater, do the following:

  1. Open a terminal window.
  2. Type sudo apt-get update.
  3. You will receive error GPG error: [server] all Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY [signature]
  4. Type sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys [signature from the above error message].
  5. Type sudo apt-get update. If there are still signature errors repeat step 4 until they stop appearing.
  6. Run Software Updater.