Prior to your system having the ability to simply restart Kubuntu without rebooting, you will need to enable this option in System Settings. Go to → → and then press the icon. At the very bottom of the window, just above the , , and buttons, there is a checkbox. Make sure to select the checkbox that says Ctrl+Alt+Backspace restarts the xserver. Press the to apply your changes. You will have to log out and then back in prior to this option takes into effect.
Once you have configured your system to allow this option, to simply restart Kubuntu, or the X Window System™, simply press Ctrl+Alt+Backspace. This will take you to a blank screen momentarily and then bring you back to the login screen (KDM).