The easy way !
The easy way is to install the KDE package from the terminal shell. Open terminal and type the following:
sudo apt-get install kubuntu-desktopEnter your password and just wait. The download should begin shortly, after the package is downloaded choose your default login manager (kdm for KDE and gdm for Gnome).
The other way!
The othe way is to install KDE with the help of the Synaptic package manager. Open Synaptic Package Manager from the Administration menu. Type your password and then click the search button. In the new window type kubuntu-desktop and hit Search. Synaptic will find only one package, right click on it and select "Mark for Installation". New window will appear with the list of the packages, which are going to be downloaded and installed. Click Apply and the downloading process will begin. After they are downloaded they will be prepared and installed. Through the installation a new window will pop up asking you to choose the default login manager. KDM is for KDE and GDM is the gnome display manager. When the Changes Applied message appears, you can safely click Close. Now you have to logout and reboot your computer. When the login manager comes back up, you have to manually select KDE as your desktop environment; otherwise you are logged in to the last environment you were using. When the login manager shows up click the Options button and select Session from the menu. A new window will be opened. Select KDE and hit the Change Sesion button. Enter your login details and anjoy the new KDE desktop :)