As stated in the disassembly guide it's easiest to rest the upper shell on the front right corner of the system due to cable length restriction. The same will hold true here. I found it easiest to lift the main PCB at a 90 degree angle resting against the upper shell. Mind you the main PCB will be resting on the Genesis controller port PCB

Try to stabilize the NES controller board so that the cables don't become disconnected. If they do, you'll have to fight through the glue in order to resolder the cable.

NES controller boards location.

This is where your defective switch and it's 6 pins would be located looking at it from a downward angle.

Once you've got the PCBs arranged in a way where they won't fall over you can begin to remove the switch. Either using a braid/pump/bridge the 6 pins that are soldered to the through holes of the main PCB and remove your defective on/off switch.

Mine has already been replaced so pictures will not depict what the board looks like without the switch.