Remove bottom housing to reveal the inside of the iMac.
Unplug all the cables from the computer, including the power cable. Lay the computer face-down, supporting the neck and base with a soft cloth under the screen.
Loosen the four Phillips #0 screws.
Carefully remove the base access panel.
Remove the four 25mm T-15 screws.
Open the housing plate.
A fixed plug connector between the logic board and upper unit will cause some resistance. Pull gently but firmly.