You won't be able to do it. The heatsink covers several chips at the same time and is a bespoke shape. You need to remove the panhead screws with the small spring underneath them and several black and chrome ones around the case. The heatsink will still be stuck to the chips by the heatsink compound which will have to be cleaned off and new applied when you reassemble. I know all this as I dismantled mine for cleaning.
iMac Pro review