When you are on the COMSOL Desktop, you can toggle between the Application Builder and COMSOL Multiphysics. On the Home toolbar, click Application Builder 
to open the Application Editor, where you can modify the user interface of the application as well as create and edit code for the application. You can also press Ctrl+Alt+A.
to open the Application Editor, where you can modify the user interface of the application as well as create and edit code for the application. You can also press Ctrl+Alt+A.Conversely, when you are in the Application Builder, you can click Model Builder 
on the Home toolbar to return to COMSOL Multiphysics. You can also press Ctrl+Alt+M.
on the Home toolbar to return to COMSOL Multiphysics. You can also press Ctrl+Alt+M.