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
Updating programs to latest neon version works (after adding the ppa, the system can now find the latest versions of the kde programs)
but installing
neon-desktop
requires additional packages that are not installable on my system apparently