Unplug the fan cable from the Motherboard.
Remove the 7 retaining screws and springs on the Heatsink assembly in the order stamped on the Heatsink and lift the assembly free from the system board.
Clean the old thermal compound from the CPU and the GPU. Apply a thin film of a quality thermal paste such as "Arctic Silver" when reassembling the unit. Leave all thermal pads in place for reassembly.