To lift the front bezel off the iMac, simultaneously:
Use your thumbs to press in the RAM arms and hold the iMac down.
Use your index fingers to pull the small bridge of material on the front bezel toward yourself.
Pull the front bezel up with your index fingers.
Once the small bridge of material has cleared the RAM arms, lift the front bezel by its lower edge just enough to clear the bottom edge of the rear case.
Lift the front bezel off the rear case and rotate it away from the bottom edge of the iMac, minding the camera and microphone cables still attached to its upper edge.
The third picture shows the top front bezel brackets and their slots cut into the top edge of the iMac's rear case.
Do not completely remove the front bezel, as it is still attached by the camera/microphone cables.
Pull the right edge of the logic board slightly away from the rear case to dislodge the rear I/O ports from their bezel.
Tilt the top edge of the board away from the rear case and lift the logic board assembly out of the rear case, minding any cables that may get caught.
When reinstalling the logic board, it is helpful to use tape to temporarily tuck the cables out of the way until the board is properly seated in the rear case.
During reassembly of the logic board, pay attention to the position of the I/O connectors. When the board is back in the case, insert a USB or Thunderbolt cable into one of the connectors to align it perfectly.