Update a Windows Phone project
With Microsoft Expression Blend Service Pack 2 (SP2), you can easily update your Windows Phone 7 projects to Windows Phone 7.1.
Create a backup of your Windows Phone 7 project before updating it to Windows Phone 7.1. Expression Blend 4 SP2 does not automatically save your project before updating it to Windows Phone 7.1, and you cannot revert a project after it has been updated.
To update a Windows Phone 7 project to Windows Phone 7.1
On the File menu, click Open Project/Solution.
In the Projects panel, right-click the project name and then click Update to Windows Phone 7.1.