****You can upgrade an existing Windows Phone project in Visual Studio in one of the following ways:****
1. In Solution Explorer, use the Upgrade to Windows Phone 8.0 command on the project’s context menu.
2. In Project Designer, change the Target Windows Phone OS Version property value to Windows Phone OS 8.0.
To upgrade a Windows Phone OS 7.1 project by using the Upgrade to Windows Phone 8.0 command in Solution Explorer by following Method:
1. In Visual Studio, open the Windows Phone OS 7.1 app project that you
want to upgrade.
2 . In Solution Explorer, right-click the project, and
in the context menu, select Upgrade to Windows Phone 8.0. The
project is upgraded to target Windows Phone 8.