Peel back the aluminum EMI tape from the two vertical 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 hard drive data cable connector from the logic board by pulling it straight up from its socket.
The SATA connector on most iMacs tends to stick in its socket on the logic board. If you are having trouble disconnecting the SATA cable, insert a metal spudger or any other thin tool into the gap between the SATA connector and its socket and twist the spudger's shaft to safely separate the two pieces.
Push the two RAM arms protruding from the access door inward and set them on the lower edge of the rear case to prevent them from getting caught when removing the logic board.
While grasping the logic board with one hand, disconnect the DC power cable connector from the logic board by pulling it straight away from its socket.
It may be helpful to wiggle the connector while pulling it.
It may be necessary to hold the Phillips head screw sockets with a Phillips screwdriver while unscrewing the four Torx screws near the corners of the processor.