Open kate (or another editor) as admin:
kdesu kate /etc/apt/sources.list
add this line:
deb http://kubuntu.org/packages/kde-356 edgy main
Save and close.
These packages have been digitally signed using Jonathan Riddell's key.A copy of the key is also kept on people.ubuntu.com for verification.
To add this key do:
sudo apt-key add kubuntu-packages-jriddell-key.gpg
sudo apt-get update
sudo apt-get dist-upgrade
Thats it 🙂