Last week, Microsoft released an optional update for Windows Phone SDK with two new Windows Phone 7.8 emulator images to our existing Windows Phone SDK installations.The update does not change our existing Windows Phone OS 7.1 emulator images, and the update supports both the Windows Phone SDK 7.1 and the Windows Phone SDK 8.0 ,but there are no new APIs available to Windows Phone apps in Windows Phone 7.8 Most importantly, any Windows Phone apps that you build using the Windows Phone SDK still target and run on Windows Phone 7.5. This update simply makes it easier to test how your apps appear on devices running Windows Phone 7.8.
What’s exactly is include in this update Two new Windows Phone 7.8 emulator images to your existing SDK installation
-Windows Phone 7.8 emulator: This OS image emulates Windows Phone 7.8 running on a 512-MB device -Windows Phone 7.8 256MB emulator: This OS image emulates Windows Phone 7.8 running on a 256-MB device -If you’re running a Windows Phone SDK 7.1 installation, the update will also download and install the Windows Phone SDK 7.1.1 update onto your machine as part of the update
You can download this update from here : http://www.microsoft.com/en-us/download/details.aspx?id=36474