Permanent change in MainWindow from a child window with out creating an instance of MainWindow!

[Asmaa]

I have many windows. I want to change in the Mainwindow from a window (window3). Make a label visible in

MainWindow on a button click in window3.

I don't want to create an instance of MainWindow, because that doesn't keep changes in the parent Mainwindow.

How can I do it, kindly?

Thanks in advanced.

[Asmaa]

Any idea, please?