Re-orient your iMac so it sits upright on the stand.
Insert a plastic card up into the corner of the air vent slot near the top of the rear case.
Push the card toward the top of the iMac to release the front bezel latch.
Pull the front bezel away from the rear case.
Repeat this process for the other side of the front bezel.
It may be necessary to apply several layers of duct tape to the top of the access card to aid in releasing the latches.
If the bezel refuses to release, try pressing the lower edge back onto the rear case and repeat this opening process.
Alternatively, you can use a strong magnet by holding it to the front top left/right corner of the display. You will hear a snapping sound when the hatch is released.
Peel back the two EMI tape strips from the left and right edges of the display.
During reassembly, it is helpful to use several small strips of tape to hold the EMI shielding along the left and right edges of the display footprint out of the way before lowering the display into the rear case of your iMac.
Disconnect the DC-in cable by simultaneously depressing both locking arms and pulling its connector away from the socket on the logic board toward the top of your iMac.
After both locking arms pass their retaining tabs, it is helpful to push the arms toward the top edge of your iMac while wiggling the connector.
Disconnect the IR/camera cable from the logic board.
This connector is especially wide and prone to breakage.
To aid in removal, it is helpful to use one hand to pinch the cables against a spudger and pull up toward the top edge of the iMac while pulling up gently on the cables with your other hand.
Continue rotating the board toward yourself until you have enough room to reach the SATA connector (shown in red).
Insert the blunt end of a metal spudger between the SATA connector and its socket. Twist the shaft of the metal spudger to separate the connector from its socket.
Disconnect the SATA cable from the logic board.
Lift the logic board out of the rear case by its edges, minding any cables that may get caught.