1. Open Visual Studio
  2. File -> Open -> Project/Solution
  3. Browse to the .sln file you would like to convert from a farm solution to a sandboxed solution (or vise versa) and click “Open”.
  4. View -> Properties Window
  5. Under the “Properties” section. set the “Sandboxed Solution” attribute to “True” to convert to a sandboxed solution, or set the property to “False” to convert to a farm solution.
  6. Build -> Build Solution.
  7. Build -> Deploy Solution. Finished!