How to Replace the Fan on a Power Supply

OK, try this :

As a rule, don't mess around with anything that you don't need to touch - the capacitors in a power supply are dangerous, and can kill you if you discharge them accidentally.  So if you're unsure then yes, take it to a technician or (and this may be cheaper) buy a new Power Supply Unit.

That said, if you still want to do the job then disconnect the power supply unit from everything, especially the mains(!) and leave it standing for 2 or 3 days minimum - I usually leave a PSU standing for a week unless I have a damn good reason and this way the capacitors should lose most of their stored power.  There are ways to speed up the process by discharging the capacitors by hand, but don't try messing around with them.

When you're ready, use an insulated screwdriver to undo the screws holding the power supply together - some of the screws may be under the warning labels - and lift off the top cover.  The fan itself is usually held in place with 4 screws, 1 in each corner, and you need to remove these to remove the old fan.  The fan usually draws its power from 2 exposed pins on the circuit board which it plugs into and covers - this can be pulled off because it shouldn't be soldered on - but take note of which one the red wire connects to.

The replacement fan goes in the same way - screw it back into place and connect its power cable the same way around as the old one, then replace the top cover and screw into place.  Replace the power supply back into the case and reconnect it.  It may be worth plugging it back into the mains ONLY at first and turning it on, because if something does go wrong, you don't want to blow your motherboard and drives.  If that works then disconnect from the mains, replace all cables to drives, motherboard etc and you're ready to roll.

Like I said earlier  If you're at all unsure about doing this then get a technician to do it for you - they've got the training, and hopefully they're insured.....