After installing KDE Plasma desktop in Ubuntu, it automatically takes over your default Ubuntu LigntDM login screen and displays KDM instead. You can easily switch back to LightDM by running a simple command in terminal.
Follow the given steps:
Step-1 So first thing you need to do is click Dash Home icon present on the top of Ubuntu Unity Launcher, type terminal in search box and open it.
Step-2 Second step is to type or copy-paste the following command in the terminal and hit ENTER
sudo dpkg-reconfigure kdm
Step-3 Now there should popping menu on your screen asking you to select default display manager. Use your arrow keys and select LightDM instead or KDM.That’s all with the settings.
Finally restart your PC.