Leave the cpu retainer clip in the locked position. Release the heatsink retainers clips and give the HS a slight clock then counterclock wise twist. Don't get too rough. As you feel it coming loose increase the motion. when you have done this several times give the HS a gentle tug up from the chip. If it doesn't come loose just go back to the twisting motions some more and try again.
Don't just snatch the heatsink up or you may damage the cpu chip itself.