Create a new project for windows phone 7.1 and then import all your files of the application to that project. Make sure you do not import the WMAppManifest.xml file. That file needs to be the actual one from 7.1 project.
Now compile the project. If you any making use of any SDK calls that aren't in 7.1 then you will need to address them! That should not be tough i suppose. There are workarounds for most of them. You can post those errors here for more help.
