Edit source prefab : change position or add new gameobject

I have already created a prefab of my commonly used object. If I wish to add a new game-object or component to the hierarchy of the prefab, so that all instances referring to it would be updated, how do I go about doing that?

Some things you can add by clicking on the prefab and adding items using the Component menu. For more involved changes, drag the prefab into the scene, make whatever changes are necessary (ignore the message about breaking the prefab link), and when done, select Apply Changes to Prefab from the GameObject menu. Then delete the prefab from the scene.