Open a terminal and enter
sudo apt-get update
Then once you’re sure all your repositories are up to date enter
sudo apt-get install edubuntu-desktop
Doing that should get you the Edubuntu desktop, as installing the other desktops from a default Ubuntu install are pretty much the same, just with the name of the desktop, such as
sudo apt-get install xubuntu-desktop
to install Xubuntu on top of a default Ubuntu install, or
sudo apt-get install kubuntu-desktop
to install the Kubuntu desktop on top a default Ubuntu install