When remove the CPU heatsink it's best to heat up the heatsink with a blow dryer so the thermal paste heats up. This way you don't rip the CPU out of the motherboard socket.

Remove the CPU heatsink screws.

Make sure you do the normal CPU unscrew method. For example, start with the upper left, unscrew just a bit, then go to the lower right and unscrew just a bit. Next jump to the lower left, unscrew just a bit, then next the upper right. Continue this until you can remove it.

If the heatsink doesn't easily left up, heat it up more with a blow dryer. To make sure you don't rip out the CPU. Ripping out the CPU from the motherboards socket could damage the CPU and the motherboard.