How best to navigate to a new page while in a sub frame

I am brand new to WPF (and .NET in general) and am writing code based on online samples and tutorials.  Please bare with me if I sound ignorant of general knowledge since I only worked on native kernel code for the past 7 years.

Anyway, I am trying to write a desktop app that quires navigation and the best model I have so far is a Frame embedded in a NavigationWindow.  There are times when only a portion of the GUI changes and other times when the whole thing changes.  One of the state transitions is triggered by an event in a page hosted by the internal frame (like a button click) but I want to navigate the entire window to a different page.  So far, I haven't found any examples doing this.  I got it to work by pretty much locating the main window global variable.  My code in the frame page is:

private void FramePageButton_Click(object sender, RoutedEventArgs e)
{
    NavigationWindow mainWindow = (NavigationWindow)App.Current.MainWindow;
    mainWindow.Navigate(new MainPage2());
}

I am not a big fan of this since I think it breaks OO abstraction.  The contents of the frame page should not be aware of the main window and how it transitions between states.  A better approach is probably using a regular C# event/delegate but I am not sure how to hook it up nicely.  The page in the frame gets loaded automatically when the app starts because it is declared in XAML.  I guess one thing I can do is to instantiate that page manually in the host page's constructor and tell the frame to navigate to it.  This way I can attach a callback to the page's delegate.

So, does anyone know what's the preferred approach?  This feels to be a pretty standard pattern for navigation apps.  Thank you very much.