I'm running Kubuntu 16.04 64-bit with: KDE Plasma Version: 5.5.5; Qt Version: 5.5.1; KDE Frameworks: 5.18.0.
I attempted to get KDE 5.7 by running the following commands:
sudo add-apt-repository ppa:kubuntu-ppa/backports; sudo apt update && sudo apt full-upgrade
Reading package lists... Done
Building dependency tree
Reading state information... Done
Calculating upgrade... Done
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.