Add the Bitcoin Ubuntu PPA repository in your Ubuntu
sudo add-apt-repository ppa:bitcoin/bitcoin
Update package lists
sudo apt-get update
Install bitcoin-qt
sudo apt-get install bitcoin-qt
Just copy and past the code into terminal and you should have the latest version of bitcoin-qt.
If you prefer not to use a PPA you can download the latest linux client from here:
http://bitcoin.org/en/download. Just decompress the .tgz file and you've got the latest version of bitcoin-qt.