How to do this?
If I logout and login - there is no options to choose KDE.
Also if I open settings > Details > Users - there is no option to choose KDE
I have followed the tutorial, everything was installed without errors, during reboot ubuntu logo changed with kubuntu, but it still logins to GNOME 
First make sure that the sddm is installed on your system. Most likely it is already but just in case run:
$ sudo apt install sddm
If the sddm is already installed than try to reconfigure it in order to make sddm the default display manager:
$ sudo dpkg-reconfigure sddm
This will ask you to choose your default display manager. Select sddm:
Confirm that sddm is selected as default:
$ cat /etc/X11/default-display-manager
/usr/bin/sddm
The easiest now is to simply reboot your Ubuntu system and make sure that you have selected Plasma desktop as a default desktop environment:
Hope this helps…
Lubos
