At the topside of the cpu socket, you'll see the socket is longer than the CPU, a raised section. At this end you should notice three 'plugs' that a metal clip from the heatsink is connected to, and usually theres a little hole for a screwdriver in it (to make it easier to apply preassure). This clip is what is holding the heatsink on (if you cant see the clip or this makes no sense looking at your setup, ignore - you might have a different setup).
Touch the back of your case to discharge any static you might be carrying!
Press down on this lever - the aim is to take the metal clip out from under the plugs, to unclip it. Once this side is unclipped, unclip the other - not necessarily by pressing down, sometimes lifting the heatsink off at an angle works easiest. Careful - watch your motherboard doesnt bend too much, it could get damaged. Its tempting to do this while mobo still in the case, but dont, it really is a LOT easier (especially when putting heatsink back on) to take the motherboard out and place it on a non-static, flat surface (preferably also put a antistatic bag to put mobo onto). Also make sure your Heatsink doesnt scrape on the CPU top, or get jarred around much.
I guess i'd better point out that when putting new heatsink & fan on, apply some thermal paste/grease/compound (whatever you want to call it) onto the kernel... basically look here:
http://www17.tomshardware.com/howto/01q1/010115/pcsystem-06.html
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