This worked for me and might not work for your.
Try it at your own risk!
$ wget -qO - 'http://archive.neon.kde.org/public.key' | sudo apt-key add -
$ sudo apt-add-repository http://archive.neon.kde.org/user
$ sudo apt-get update
$ sudo apt-get install neon-desktop
Upgrade everything. Repeat if necessary, until everything is really up to date and you have no held-back packages.
$ sudo apt-get update
$ sudo apt-get upgrade
$ sudo apt-get dist-upgrade
Oh. And I had troubles with KDE games, so I remove as much as I could, by doing:
sudo apt remove kmahjongg kmines kpat ksudoku libkf5kdegamesprivate1 libkf5kdegames7 libkf5kmahjongglib-data libkf5kmahjongglib5 kdegames-mahjongg-data-kf5 kdegames-card-data-kf5 libgamemode0 libgamemodeauto0 libkf5kdegames-data
and to make sure all dependencies are finally correct:
sudo apt-get dist-upgrade