Remove the square black sticker.

Using the tweezers remove the dust which is outside of the heatsink's ventilation box. Then, using an air compressor or a vacuum, dust the box off until you are able to clearly see trough it.

Finally, remove the old thermal paste from the heatsink using the ArctiClean #1 and repeating the same 60-seconds operation you carried out on the CPU and GPU. Clean it until you can not see any gray stain on the cloth.