When updating your Linux pc's you may have noticed that ros is not properly updating anymore and gives errors similar the following...
Err:5 http://packages.ros.org/ros/ubuntu bionic InRelease
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY F42ED6FBAB17C654
To fix this you need to add the new GPG key and delete the old one. Run the following commands...
sudo apt-key del 421C365BD9FF1F717815A3895523BAEEB01FA116
sudo apt-key adv --keyserver 'hkp://keyserver.ubuntu.com:80' --recv-key C1CF6E31E6BADE8868B172B4F42ED6FBAB17C654
Then run the following to make sure it's fixed
sudo apt update
It should finish with "All packages are up to date."
Here is a link for more info on why this was required. https://discourse.ros.org/t/new-gpg-keys-deployed-for-packages-ros-org/9454