Once you separate the heat-sink from the motherboard, you will need to clean and re-apply thermal compound. Do not proceed unless you are prepared and you have the necessary materials.

Using both hands, gently lift the heat-sink and fan components from the motherboard.

Old thermal compound can dry up and adhere the heat-sink to the motherboard like glue. This may make removing the fan and heat-sink difficult.

Do not re-seat the fan and heat-sink without cleaning and re-applying the thermal compound. For more information, please refer to this guide.