First of all, get a new fan from taobao at 15 RMB, it contains the heat sink for the CPU and chipset:
Taking away the keyboard is relatively easy, and you can see the fan now, but then it could not be replaced yet since the CPU and chipset is on the other side, unluckily.
So you have to remove this board from the case...
You finally see the whole fan/heat sink assembly:
Take away and replace, done!
Finally, a video to tell you how to disassemble completely: youtube