Exchanging the fan is possible. It involves disassembling your tower, unmounting the power supply (unplugging all the cables first) and then opening the housing of the power supply with a small screwdriver.
I would recommend to do this before you go and get a new fan because you need the fan's specifications. The fan is located inside the power supply.
Again, make sure you really have disconnected all cables from the power supply. After having it opened, take care not to touch anything inside. Some of the components may still have current (the big condensors especially).
Use a ruler to measure the diameter of the fan. In my old PowerMac the fan's diameter was 92 millimeters (a real big monster fan). Also measure how thick the fan is.
The fan has 2 cables, red and black. It takes 12 Volts DC (please verify on the fan's label if any).
Now, after you know the fan's specifications, put the power supply together again and assemble the tower, then get a good replacement fan. I recommend the Papst fans. I replaced the one in my old PowerMac and it made a biiig difference.
You should get a good replacement fan in an electronics store. Maybe a mail order shop if you can't find it in your town. Buy a replacement fan which has the same diameter and which is as thick as the old one. Of course, it must have the same voltage. A good fan may cost 30 $/Euros.
Having the replacement fan, you need to unmount the old fan from the power supply's housing. The 12V supply wires are probably soldered to the supply's printed circuit. You can cut them immediately at the old fan so that they remain quite long. Then, connect them to the new fan's wires. If you want to do it perfectly then solder them together and insulate them with heat shrink tubing. However, a luster terminal (insulating screw joint) will do as well.
If the new fan has 3 wires, one of the wires provides the speedometer signal (rpm sensing). Normally, this is the blue wire, but it depends on the fan's brand. Simply don't use it, cut its end and insulate it a bit.
The new fan will fit exactly into the mounting holes of the old one if you bought the right diameter. So the rest will probably be easy.
Good luck
