In older versions of Unity there is no prefab mode, and you edit a prefab directly in the hierarchy. After making changes, click ‘Apply’ in the inspector to save the changes and update other prefabs throughout your game.
Any changes made to the prefab while in prefab mode are automatically saved to the prefab asset. And those changes will also update in all instances of that prefab asset in the project. Including other prefabs that include this modified prefab as a nested prefab.
Exit Prefab Mode by clicking on the ‘Scenes’ button in the Breadcrumb Bar. You can also use this Breadcrumb Bar to go to the parent prefab if you are editing a nested prefab. Alternatively, click the back arrow at the top of the Hierarchy Window to exit Prefab Mode.