You might have to tap the face of it with a hammer a couple of times to both loosen it a straighten it. There is just enough slop in the fit that sometimes they get a little crocked and they won't slide right off. When it is straight it will slide right off. If it does not just tap it on the face from one side to the other until it is free and straight and then it just pulls off. Make sure that you put it on with the little nub towards the outside and align the tooth with the slot on the crank, install the shields correctly (inner and outer) otherwise it is pretty much fool proof. What is wrong with the old one that you are replacing it and are you going to do the seals while you are in there?

Mark