It is a bit tricky and the service manual is a bit confusing. The fan itself is enclosed in a heat exchange assembly and the whole assembly has to be replaced. If you are in luck you can purchase the whole assembly from china for 20$ including P&P. I purchased mine from wishingdeals on ebay their contact information is as follows:
The plastic parts you disassemble are fragile so you should take care. You begin by taking off the rubber pads on the screen (they don't seem to come off but they do) beneath them you will find the screws. Unscrew them. By doing it you can take the screen off the hinge and that will allow you to unscrew the hinge itself, slip off the keyboard and disconnect the keyboard and screen connectors. Now you have access to the motherboard and the screws that fasten the plastic cover/frame. Remove the plastic cover/frame (a bit tricky) and then remove the motherboard. You may need to extract the CPU before the whole procedure. You have access to the cooling assembly. Disconnect the electrical fan connector from the board and replace it with the one in the new assembly. Assemble the computer and it should be silent most of the time.
Excluding the battery, the fan is one of the most abused parts of the computer, adding to the fact that the fans have a sleeve bearing and not a ball bearing (cost savings), you should clean them regularly from dust for its the major cause of premature fan failures.
Hope this short explanation solves the problem for others.