To enable global menus open System Settings, go into the Application Style category, and in the Widget Style settings you will find a tab called Fine Tuning. On this tab you can find the new Menubar options. You can change to either a Title Bar Button, which will tuck the menu into a tiny button into the window decoration bar at the top, or the Application Menu widget, allowing the associated Plasma panel to supply the menu in a fixed location.
To apply the change, your applications need to be restarted, so ideally you’ll simply log out and back in again.
To add an Application Menu to Plasma, simply right click on the desktop and add the Panel called Application Menu Bar.
Enjoy your new Plasma 5.9 with global menu bars!