Команды для установки: (То что в квадратных скобках- просто коментарий)

Install from Kubuntu with Plasma 4

If you are running Kubuntu with Plasma 4 you can install Plasma 5 packages with these commands. This will replace your Plasma 4 desktop and much of the software. No upgrade will be supported and you may have to reinstall to revert back to Plasma 4.

sudo apt-add-repository ppa:kubuntu-ppa/next
sudo apt update
sudo apt install kubuntu-plasma5-desktop [it will ask if you want to set use the new SDDM as login manager]
sudo apt full-upgrade
sudo reboot [save any work first]