First add the following repo

sudo add-apt-repository ppa:kubuntu-ppa/backports

Make sure you are updated and upgraded

sudo apt-get update && sudo apt-get dist-upgrade

And if needed do a reboot

sudo reboot

Install the KDE desktop

sudo apt install kubuntu-desktop

Log out, select Plasma and log back in.

https://computingforgeeks.com/how-to-install-kde-plasma-desktop-on-linux-mint-19/