Here are the steps to replace the power supply in your computer.
1) Turn off your PC
Unplug the computer, and remove the side of your computer’s case.
2) Unplug the power cables from the motherboard .
There are two power supply cables that plug into the motherboard. Some video cards also require power from the power supply. In this case, you also need to remove the four-pin connector cables that look just like the ones plugging into the CD drive .
3) Unplug the power cables from the hard drives and the CD/DVD drives as well as any other places on the motherboard.
Motherboards usually include small four-pin connectors for controlling switches and fans. Your old power supply will most likely have some extra cables that don’t plug into anything.
4) Remove the four screws that hold the power supply to the computer’s case.
Be careful not to remove the screws holding the power supply’s internal fan. To see which screws are which, try loosening the screws slightly and wiggling the power supply from inside the case, the screws that hold the power supply in place are generally closer to the outside edge of the computer’s rear. The screws that hold the fan are usually closer to the fan’s edge.
5) Next, lift out the power supply.
Place the new power supply in the old one’s place, and tighten the screws, then reconnect the cables to the motherboard, the drives, the fans, and the power switch. Be sure you are plugging the cables in correctly or you may damage the plug in sockets
Remember, some cables won’t connect to anything on the motherboard; since they’re for future add-ons.
6) Reconnect the power cord and plug your computer back in.
Put the case side back on, then turn the computer back on.
If all goes well, your desktop computer should be back up and running.