Lay the iMac display-side down on a flat surface (putting a cloth under the screen to protect it from bumps).
Loosen the three Phillips screws securing the rear panel to the iMac.
These screws are captive in the iMac. The center screw will stop turning after about 3.5 turns and the outer screws will stop turning after about 5.5 turns. Do not try to remove these screws from your iMac.
Remove the following three screws securing the inverter to the chassis:
Two 7.9 mm Phillips.
One 26 mm Phillips.
When reinstalling the inverter, be careful to route the purple and black cables around the screw mount. It is very easy to pinch the cable and damage the insulation.
Slide the inverter toward the edge of the mid-plane, then lift it out of the iMac.
Be careful to not pry up against the RAM brackets.
When reinstalling the inverter, press the blue and white cables down in the edge of the midplane before lowering the inverter down into the midplane. The black cable can be routed near the hard drive thermal sensor cable.