There are four screws with springs on them which have to be removed in the order embossed on the heatsink. (red circles)

Also make sure to unplug the fan very carefully. (pink rectangle)

Remove the heatsink by lifting it out to the left side as it is fixed on the right side.

The cpu DIE lies bare in front of you - this is a very sensitive part. This is why the heatsink is screwed on in a particular order.