There does not appear to be the capability to switch between the design view and the FXML source in JavaFX Scene Builder. Are there plans to provide this capability at some future point? Of course, the application should validate and synchronize changes made in either view.
You can make a feature request against the design tool (DTL- prefix) at:
In the meantime just edit the source in your favorite editor and save it and if the file is also open in SceneBuilder, SceneBuilder will pick up any changes. All major IDEs now have support for completion based editing and attribute validation on fxml documents.