Flip your iMac over and lay it stand-side down on a flat surface.
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.
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.
A hard drive thermal sensor may be present on some models. If your logic board is missing the hard drive thermal sensor socket highlighted in red in the first picture, skip this step.
Pull the hard drive thermal sensor connector away from its socket on the logic board.
Pull this connector parallel to the face of the logic board toward the top edge of the iMac.
Use a spudger to pry the thermal sensor and adhesive off the hard drive and install it in the same location on the replacement drive.
Press the hard drive bracket down to free it from the rear case, then rotate the top of the drive toward yourself.
Pressing the top of the hard drive bracket down to release it from the rear case requires a substantial amount of force, so we recommend laying your iMac stand-side down on a table to avoid knocking it over.
If you have trouble releasing the hard drive bracket, it can be removed from the hard drive, by removing the two T8 or T10 Torx screws securing the bracket to the drive.
Lift the hard drive off its lower pins and pull it out of the rear case, minding the power supply cables that may get caught.
When reinstalling your hard drive, be careful not to push the two rubber grommets through their holes in the chassis. Retrieving them would require removing the logic board.
The next few steps bring your hands close to the exposed face of the power supply. Do not touch the face of the power supply to avoid a high voltage shock from the many large capacitors attached to the board.
Unplug the power supply cable bundle from the logic board by simultaneously depressing the locking mechanism (toward yourself) with a spudger and pulling the connector away from its socket.
I wanna try this because my imac no longer turns on, but i dont know if its my power supply. I press the power button and it turns on barely for a second and then turn off completely. it makes the sound when a mac turns on and the screen turns on for a sec…..what should I do?