I made an app to WP8 and I used the page navigation like NavigationService.Navigate(new Uri(...))
and worked well.
But now I\'m trying to develop a WP8.1 app an
For Universal Apps (Windows Runtime), you need to use the Frame.Navigate(...) overload, with the type of the page you're navigating to.
To navigate to MainPage.xaml, you can use Frame.Navigate(typeof(MainPage));
I'll recommend reading Quickstart: Navigating between pages (Windows Runtime apps using C#/VB/C++ and XAML) (MSDN)
You can navigate using
Frame.Navigate(typeof(MainPage));