How do those cranes that build skyscrapers that go up with the building get back down after it's completed?
The top sections of the mast beneath the crane are removed one at a time and lowered to the ground. The top section of the crane has hydraulic rams that lower it down one mast section at a time. It is hard to describe without a diagram or photos. See photos at attached link. In some design, the jacking portion is at ground level. The crane jacks itself up so a 20' section of the mast can be unbolted and slide out. Then the jack is lowered so the process can be repeated. See second link for photos and a more detailed explanation.
little pieces by welly chopper