Its name is "Prefab Editor". By using this, you can edit Prefab which could not be edited much unless you place it on Scene.

With this editor extension, the Prefab selected in the Project view is displayed in the Prefab Editor Window. The appearance on the Window references Hierarchy as it is. And what you can do,

• Display 3 layers or less of Prefab, and when selected, it also appears on Inspector
• Parent-child relationship of GameObject can be edited
• Fit in a GameObject placed on another Prefab or Scene
• Rename it by double clicking
• Right click to create and delete GameObject
• etc ...

In this way, you can do almost the same thing as on Hierarchy without having to place it on the Scene from the Project view.

Please double click on the package that can be downloaded from the link below and import it to Unity.

https://github.com/yukiringo/PrefabEditor/raw/master/prefab_editor.unitypackage

Window -> Prefab Editor

Then select the Prefab you want to edit in Project view. I think that the selected Prefab is displayed on the Window of Prefab Editor as if it was placed in Hierarchy.

After that, it is OK if you edit it in the same way as doing with Hierarchy.