You only need to unplug this cable from one end. You can just fish the cable through the hole, then you can separate the motherboard from the console. There are small plastic guides holding this cable in place; these guides are pretty fragile but it's not a big deal if you accidentally bend or break them.

There is a safer way to unplug this cable from the motherboard. If you look closely, there are two small slits on the back of the connector. If you look very closely, there is a tiny hole inside each slit. If you can find something small enough to fit in that hole, you can push the connector out from behind. It takes some force, but it's less risky than pulling directly on the wires.

(This advice might not apply to all motherboard revisions)

This cable is very hard to remove from the connector. It's much easier to unplug this cable from the motherboard side (in step 20) because you can push the connector out from behind. (This might only be true for some motherboard revisions.)

You can leave the power supply propped up on top of the console, like shown in the first picture. After you've unplugged the two cables in step 13, you can put the power supply back where it was to make flipping the console over easier.