if you do NOT have Libre Office installed at all try this to get it installed:
sudo apt-get install libreoffice
So, I'll use my situation in this example for upgrading to Libreoffice 4.1.x, in my case simply 4.1 (I started out with Libreoffice 3.5 which comes installed with Ubuntu 12.4)
first, in terminal type in.. (I added the 4-1 myself which my help was 4-0 since 4-1 is now out!!)
sudo add-apt-repository ppa:libreoffice/libreoffice-4-1
then type in terminal..
sudo apt-get update
Finally, type this command in to upgrade your system accordingly to work with LibreOffice
sudo apt-get dist-upgrade
It will take some time to install and you'll see alot of file and package updates taking place but it worked quite well for me.