title: How to revert/downgrade Windows Phone 8.1 app to C#5, I have accidentally upgraded to C#6
body: <p>How to revert Windows Phone 8.1 app to C#5, I have accidentally upgraded to C#6 and the project is not compiling anymore.  Does anyone know how to revert to C#5? Thanks.</p>
  
  <p>Long story: I wanted to try out a new feature (the null propagation in C#6 but this broke everything ^^) </p>