I have KDE Plasma 5.5 installed on Ubuntu 16.04, which is pretty ancient at 1.5 years old, but just the other day they've released 5.10. Ubuntu Software store says 5.5 is the latest version (yeah right, tell me another one).
So what can I do to get on the latest KDE? there are plenty of bugs in 5.5 that I would like to escape from.
EDIT: Looks like it may be worth upgrading Ubuntu as well.