Your CPU heatsink should be held down by four push clips. Each one has an arrow. Turn it 1/4 turn in the direction of the arrow and they'll unlock. Then just pull the clips up and out. Clean off the old thermal paste with rubbing alcohol.
The CPU is held in place by a retaining plate, which is held down with a clip. Pull outward and move the clip up, and pull out the old CPU.
Place the new CPU in the socket. It only goes in one way, lining up with the two little nubs.
Flip the retaining plate over the CPU and push the retaining clip into place. It will take a little force. Just be sure to place it under the clip to hold it in place.
Place a bead of thermal paste (slightly smaller than a pea) in the center of the CPU.
Check your heatsink and be sure it is all set to be installed. The clips should be pulled upwards so it is fully extended.
Then turn them so the rear of the arrow is closeest to the heatsink.
Place the heatsink over the holes. Imagine the clips are numbered 1,2,3,4 clockwise around the heatsink. Press down on opposing corners 1 and 3 until you hear a click. Then do the same for corners 2 and 4.
Any other questions?