Unity 5: How to enter prefab mode?

Hi, so I want prefabs in my game and I want to enter prefab mode but I can't. The Unity Manual says:
"There are many ways to begin editing a Prefab in Prefab Mode, including:

• Double-clicking it in the Project window

• Using the arrow button next to the Prefab in the Hierarchy window

• Clicking the “Open” button in the Inspector window of a Prefab Asset"

But I can't perform any of these actions: Double-clicking does nothing, there is NO arrow button and when I click "Open" nothing happens. Do note that prefab objects do have blue text in the hierarchy. Please help. Screenshots below.
Unity 5.6.4f2, Windows 10 64-bit

Screenshots: