HOW TO change the StartPage in Windows Phone 7 Application in Visual Studio 2010 [C#.NET]

Expand theĀ Properties folder into Solution Explorer of the VisualĀ C# Windows Phone Application Project and Open the WMAppManifest.xml file.
Change the NavigationPage with you new StartPage filename (case sensitive).

    <Tasks>
      <DefaultTask  Name ="_default" NavigationPage="MainPage.xaml"/>
    </Tasks>

Did my solution solve your problem?