EDIT: If you are using QtDesigner, on the property tab, there is an editable property called windowTitle which can be found under the QWidget section. The property tab can usually be found on the lower right part of the designer window.
I know this is years later but I ran into the same problem. The solution I found was to change the window title in main.cpp. I guess once the w.show(); is called the window title can no longer be changed. In my case I just wanted the title to reflect the current directory and it works.