duplicate prefab: disconnecting

Stupid question here (well, stupid person here!)

Let's say I have 3 levels in a game. Each level has a terrain based on a ground prefab. I want to change the prefab texture in each level.

I duplicated my prefab object "ground" and labeled them "ground1", "ground2", "ground3".

But, when I change the texture on one of those prefabs, it changes it on all 3. I'm not talking about the instantiation of the prefab in the scene, I mean it changes the prefab object.

I want 3 separate, unrelated prefabs--so how can I easily copy everything over from one to a second prefab (keeping all settings except the texture)? I can't seem to disconnect ground2 and ground3 from each other!