First things first, you will need to edit the /etc/apt/sources.list file:
sudo gedit /etc/apt/sources.list
and add the following line at the end of the file:
deb http://dl.google.com/linux/deb/ stable non-free
Now download the key:
wget -q -O - https://dl-ssl.google.com/linux/linux_signing_key.pub | apt-key add -and finialy update:
sudo apt-get update
You're good to go!