Lift the logic board out of the midplane. Careful, there is a light guide on the lower left that can tear if you are not careful. This guides the light to the power-on light at the front of the iMac.

When reinstalling your logic board, be sure to clean and apply a new layer of thermal paste to the U3 chip (highlighted in red) located on the underside of your logic board as well as its heat sink attached to the midplane.

We have a thermal paste guide that makes applying thermal paste a snap.