For Ubuntu 11.10 and later
Launch your Terminal
sudo apt-get update sudo apt-get install lubuntu-desktop
For Ubuntu 10.04
Launch your Terminal
sudo add-apt-repository ppa:lubuntu-desktop/ppa sudo apt-get update sudo apt-get install --no-install-recommends lubuntu-desktop
Removing other x/k/ubuntu components after upgrading to Lubuntu
Adding Lubuntu to your system will keep all the applications from your existing system. If you wish to get rid of them Pure LXDE has the details - make sure you use the correct version of the instructions. Prior to 11.10 you need simply alter the very end of the command from:
sudo apt-get install lubuntu-desktop
To:
sudo apt-get install --no-install-recommends lubuntu-desktop
This will not delete any of your own data from those applications. If you later wish to re-add them, your data will be there waiting for you