Install a PPA for backports to the current version of the OS.
In order to upgrade your current Plasma version to the latest, launch your
terminal and run following command to add Kubuntu backports repository to package manager.
Run the next command to add the PPA.
add-apt-repository ppa:kubuntu-ppa/backports
Run following package manager command to update APT repositories.
apt update
Now run following dist-upgrade command to get new plasma version on your system.
apt-get dist-upgrade