Once you have created and saved a prototype, you can edit it again by selecting the prototype in the palette, and choosing View > Edit Group (Ctrl+E). All the changes you make will be propagated to the instances you have created when you save it.

For instance, you can select the “bulb” prototype in the “samples” palette, and double click on its icon: the Prototype edition buffer is opened, and the group inspector allows you to edit its interface and behaviors.