To remove the clip holding down the heatsink, it's best to have the motherboard out of the case and on a hard flat surface to support the motherboard and so you can have easy access to the parts.
Look at the clip and see if there is a slot or spot where you can stick a small screwdriver blade into...it might have a hinged portion, it might not. What you want to do is put some thick cardboard around the base of the CPU to protect the motherboard in case the screwdriver slips. Then insert the screwdriver blade into the slot or spot and push downward. Once the clip is free from the lug on the socket, push the screwdriver handle towards the heatsink...this makes the bottom part of the clip kick outward to clear the lug. Once that happens, let up on the screwdriver and the clip will be free.
If there isn't a place to put the screwdriver blade, you're going to have to use two screwdrivers. One to push down with and the other to pry the clip outward to it clears the lug.
Cricket