Every ROOT object drawn on a pad can be interactively edited. In addition to the pop-up editor (opened from the menu obtained by right-clicking on any object), each canvas can also host an editor (opened by selecting "Editor" from the "View" menu provided by the window). To modify any object shown by the canvas, simply open the latter editor and click on the object. To find the corresponding C++ code, simply save the current canvas as a ROOT macro (that can be executed to produce the very same result later) and locate the statements corresponding to your changes.