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Check to see how its mounted and just go backwards. If it uses barb clips, get a pair of tweasers or needle-nose pliers and compress them and put them back through the hole.
If its on there, and it looks like theres nothing holding it on then its termally glued. if you REALLY want to get it off (you shouldnt need to take if off) you can try twisting it to break the seal or if you want to be really sure and a bit safer put it in a sandwhich bag (the ones with the air tight seal at thing at the top) and chuck it in the freezer till it gets cold then twist it and it should just come off and leave some crap on the gpu, just clean it off.
Any reason your taking the hsf off something that doesnt need a fan let alone a new hs anyway?
ohhh well see the heatsink was big enough so i got a fan and super glued it on there =X. i wanna take of the HS to put thermal paste on it and OC as far as i can . prolly won't do much but better then nothing. gota make use of what i have since i can't really afford a ti4200 right now >.<! thanks for the help =D