When using command sudo apt-get update, W: A error occurred during the signature verification.

Problem:

W: An error occurred during the signature verification. 
The repository is not updated and the previous index files will be used. 
GPG error: http://packages.ros.org/ros/ubuntu xenial InRelease: 
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY F42ED6FBAB17C654

W: Failed to fetch http://packages.ros.org/ros/ubuntu/dists/xenial/InRelease  The following signatures couldn't be verified because the public key is not available: 
NO_PUBKEY F42ED6FBAB17C654

Solution :

sudo apt-get clean
sudo mv /var/lib/apt/lists /tmp
sudo mkdir -p /var/lib/apt/lists/partial
sudo apt-get clean
sudo apt-get update

and then

sudo apt-key del 421C365BD9FF1F717815A3895523BAEEB01FA116

# add the new key
sudo -E apt-key adv --keyserver 'hkp://keyserver.ubuntu.com:80' --recv-key C1CF6E31E6BADE8868B172B4F42ED6FBAB17C654

finally

sudo apt-get update