I wish I can dig up some pictures for ya since a picture is better than thousand words but... anyways, you should see a clip sticking out at one end of the heatsink. Press it down and push it outward (away from the heatsink, may require a screwdriver since I don't know what kind of heatsink clip the ratail one uses). Once one side of the clip is off, gently lift the heatsink from the cpu (wait! since you're using the retail heatsink, it must have a thermal pad which is probably glued to the cpu). Unlock the cpu from the slot (lift the cpu slot lever on the side). Then gently lift the heatsink along with the cpu (if they are not glue together, remove the heatsink first before releasing the lever). If the thermal tape melted and is glued to the cpu, it may require a little force to remove heatsink from the cpu (be very gentle removing the heatsink from the cpu).