Closing an application in Kubuntu is usually a straight-forward matter, however, it may be accomplished in several different ways depending on the situation and personal choice.
For most applications,on -> in the application's Menu Bar (upper-left corner of the application window) will close the application.
on the button in the upper-right corner, on the window's titlebar, will close the application.
on the window's titlebar and choosing will quit the application.
on the application in your Toolbar's window list and choosing will quit the application.
If you prefer keyboard shortcuts, you may close an application by pressing Alt-F4 on your keyboard.
Be sure to save your work in an application, if necessary, before you close it!