The heatsink assembly includes the heatsink through which fans blow, as well as the cooling pipes and copper contact points on the other side (for the CPU and GPU). It does not include the fans (those will be removed in a later step).

To start, loosen the 6 screws holding the heatsink pressed down. The numbers don't matter, these are for when you put the heatsink assembly back in.

These screws don't come out of their holes, they are meant to stay in.

Once you're sure the screws are loosened, lift the heatsink assembly up and away from the laptop by holding it for its furthermost right and left ends. Be careful not to bend or damage anything. Place the heatsink assembly upside down.

You will notice the square and rectangle shaped contact points for cooling the GPU and CPU, respectively. They will be smudged by (most likely) old thermal paste. You will also notice these same shapes, smudged with thermal paste on the motherboard itself. These are your GPU and CPU.

This would be a good time to perform thermal paste clean up. Use your thermal paste cleaner or >90% isopropyl alcohol together with cotton swabs and cotton balls (or a microfiber cloth) to clean the thermal paste from all 4 surfaces.

Don't worry if there is some residue on the sides, thermal paste is made to be non-conductive, so your electrical components are safe.