If you want a newer Plasma it is recommend now to follow the KDE Neon user repositories. This will continuously upgrade Plasma, KF5 and KDE Applications.
For this to work you only need to add the following line as repository in /etc/apt/sources.list.d/neon.list
For this to work you only need to add the following line as repository in /etc/apt/sources.list.d/neon.list
Code:
deb http://archive.neon.kde.org/user/stable xenial main