Now re-assemble the laptop. Just reverse everything you did.

When you have screwed on the heatsink hold the fan with a finger and blast the heatsink fins with compressed air to get any dirt out. you might have to blow from both sides of the fins to get trapped dirt out.

Its very important that the fans does not spin... it will reach speeds that can destroy it.

Give case a short blast of air also to get dirt out... also blast the connectors as they are good at collecting dust and dirt