Using a soft object like a wooden tongue depressor, plastic knife or old credit card scrape the old thermal grease / compound from the CPU, GPU and Heat Exchanger. Be gentle and take your time as there is no rush and trying different angles, the dried thermal grease should just flake off. Here you could also use the Isopropyl alcohol wipes or an earbud dipped in Isopropyl alcohol to remove the dried thermal grease. It comes off easier then. If you are going to work and touch components on the motherboard, I suggest you wear an earth / grounding strap and rubber gloves.