I've replaced the fan in a few power supplies with a panaflo L1a. Works well and the operation was easy. First, remove the power supply from the case. 4 screws should do it. Some people argue that you should wait 15-86400 minutes before working on your psu to discharge all the capacitors. As long as you don't go poking things, you should be fine. Then open the power supply itself. This is usually another 4 obvious screws on the outside. The fan should look like a regular case fan with 2 wires coming from it. Check to see if there's already a molex connector at the end of the wires. If so, no splicing/soldering necessary!
If there is no molex the wires will be different colors, blue and red, black and red, white and black, etc. A good rule of thumb is black is negative/ground and red is positive/power. Unscrew the fan from the psu and snip the wires close to it so you'll have plenty of wire to work with. Twist the wires together and then tin them up. Wrap with electrical tape and screw the fan back into the psu. Reassemble the psu the way you took it apart and put it back in the case.
