Flip the green fan cover up towards you to reveal the CPU heatsink.

Lift the heat sink release lever

Remove the heat sink from the CPU and lift it out.

The heat sink may be adhered to the CPU by the the thermal pad or compound. If so gently twist the heat sink counter clockwise slightly and then pull the heat sink up and to the right.

Ensure you clean the heat sink and CPU thoroughly and apply fresh thermal compound before you reinstall the heat sink. Refer to this handy iFixit Technique if you need further information.