Are you using Kubuntu 17.04, our current release? Help us test a new bugfix release for KDE Plasma! Go here for more details: https://wiki.ubuntu.com/Testing/EnableProposed.
Unfortunately that page illustrates Xenial and Ubuntu Unity rather than Zesty in Kubuntu. Using Discover or Muon, use Settings > More, enter your password, and ensure that Pre-release updates (zesty-proposed) is ticked in the Updates tab.
Or from the commandline, you can modify the software sources manually by adding the following line to /etc/apt/sources.list:
deb http://archive.ubuntu.com/ubuntu/ zesty-proposed restricted main multiverse universe
If you are going to be testing from proposed frequently, you might try the pinning process as described on the wiki page about how to enable proposed. Otherwise, you can just
sudo apt update and then sudo apt install packagename/zesty-proposed each of the packages listed in the bug report: https://bugs.launchpad.net/ubuntu/+source/plasma-desktop/+bug/1687444.
If you do not pin, remove the proposed repository immediately after you finish installing the test packages, or you risk wrecking your system in interesting ways.
Please report your findings on the bug report. If you need some guidance on how to structure your report, please see https://wiki.ubuntu.com/QATeam/PerformingSRUVerification. Testing is very important to the quality of the software Ubuntu and Kubuntu developers package and release.
We need your help to get this important bug-fix release out the door to all of our users.