For some reason, the latest Kubuntu ships with KDE Plasma 5.5.5, which is at least two minor versions behind the latest stable release of 5.7.4.
How can KDE Plasma be upgraded?
(This question was already asked for Kubuntu 14.10 without a good answer.)
Remember that 16.04 was released in April, and the latest point release for it wasn't terribly recent. It would be pretty amazing if 16.04 shipped with a build of KDE that came out about a week ago. According to KDE's packages page, 16.04 actually came wih 5.5.3, so 16.04.1 must have upgraded to 5.5.5.
The page also says that Kubuntu can get Plasma 5.6.5 with Backports from the PPA. This isn't the latest version, but it appears that is the latest stable build for Ubuntu. I would recommend you install this one.
In case 5.6.5 isn't new enough, the Staging branch happens to have 5.7.2 for Xenial and Yakkety. You can try it out, but it isn't stable, so expect issues. I don't really recommend this, but if you want the latest you can easily get:
If you really, really, really want 5.7.4, you can build KDE completely from source. The code can be found here and build instructions here. There are quite a few packages to build, however, so I recommend you settle for 5.6.5 or 5.7.2. Unless you absolutely have to have 5.7.4 on Ubuntu, the effort needed for this is probably not worth the 0.0.2 point upgrade.
As for the reason there is no 5.7.4 on Ubuntu yet: well, take a look at the source; it's a lot. The Kubuntu team has to build all that for Ubuntu after the KDE team releases it, and make sure it's stable. Then they also have to wait for a new Ubuntu release (xx.04 or xx.10, not xx.xx.Y) to actually package it in by default.
Stable releases (5.6.5) can be found in the
backports ppa
. To install do:PLEASE BE EXTRA CAREFUL WITH THE EXPERIMENTAL PPA.
Experimental releases (5.7.2) can be found in the
staging ppa
. To install do::