All the tutorials I've seen for this are dated. I've found the Window Decorations in the system settings, but there doesn't appear to be any way to change the buttons from the right, where they are by default, to the left, more like in Ubuntu for a typical window. I'm running 16.04
KDE Plasma 5 button places
By KDE Window Decoration Handbook: (KDE System Settings > Application Style > Window Decorations > Help - lower left corner help !)
This tab allows you to customize the button location on the titlebar. You can drag buttons into the titlebar, remove them or drag around the buttons until you have the order that makes you comfortable.