The fourth point release update to Kubuntu 16.04 LTS (Xenial Xerus) is out now. This contains all the bug-fixes added to 16.04 since its first release in April 2016. Users of 16.04 can run the normal update procedure to get these bug-fixes. In addition, we suggest adding the Backports PPA to update to Plasma 5.8.8. Read more about it:
Plasma 5.8.8 LTS and Krita 3.3.2.1 now in Xenial Backports PPA
Warning: 14.04 LTS to 16.04 LTS upgrades are problematic, and should not be attempted by the average user. Please install a fresh copy of 16.04.4 instead. To prevent messages about upgrading, change Prompt=lts with Prompt=normal or Prompt=never in the /etc/update-manager/release-upgrades file. As always, make a thorough backup of your data before upgrading.
See the Ubuntu 16.04.4 release announcement and the Kubuntu Release Notes.