Plasma 5.5.3 together with Frameworks 5.18 has arrived to Netrunner via the backports ppa.
Since backports is already enabled in Netrunner 17, you can simply upgrade via the muon updater or command line:
sudo apt-get update
sudo apt-get dist-upgrade
For more info read here:
http://ubuntuhandbook.org/index.php/2016/01/install-kde-plasma-5-5-kubuntu-15-10/
For any issues please report in our forums.