I think the answer is: the opposite of how it got up there. :-)
Seriously, though, the internet tells me that you use the large crane to lift a smaller crane up, which is used to disassemble the large crane lower the pieces down. And then you bring up an even smaller crane to disassemble and lower that one. And the third one is small enough to be taken apart by hand and the pieces taken down via the inside of the building.
I know for shorter buildings they would use a crane from the ground, but I haven't seen this done on high rises before.
Seriously, though, the internet tells me that you use the large crane to lift a smaller crane up, which is used to disassemble the large crane lower the pieces down. And then you bring up an even smaller crane to disassemble and lower that one. And the third one is small enough to be taken apart by hand and the pieces taken down via the inside of the building.
I know for shorter buildings they would use a crane from the ground, but I haven't seen this done on high rises before.