When you are in the COMSOL Desktop, you can toggle between the Application Builder, Model Builder, and Model Manager. In 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+Shift+A.

Conversely, when you are in the Application Builder, click Model Builder in the Home toolbar or press Ctrl+Shift+M to return to the Model Builder, or click Model Manager () in the Home toolbar or press Ctrl+Shift+J to return to the Model Manager.