Update linux 14.04

apt-get update

apt-get upgrade

reboot

W: GPG error: http://extras.ubuntu.com trusty Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 16126D3A3E5C1192

sudo gpg --keyserver pgpkeys.mit.edu --recv-key 16126D3A3E5C1192
sudo gpg -a --export 16126D3A3E5C1192 | sudo apt-key add -
sudo apt-get update