Power supplies are needed because your computer needs different voltages than what comes straight from the outlet. Disconnecting and replacing a power supply is simple because it's just piece of hardware.
1). Shut down your computer and unplug the cable from the wall and the back of your computer. Open the computer case. This may involve unscrewing the access door from the back of the computer, pushing a lever or just sliding the door off.
2). Unplug the power connectors from all of the drives and the motherboard. The connectors can be removed by either pulling them straight out or by pushing a release button first and then pulling the connector out. Some motherboards have two sockets where the power supply is plugged in.
3). Unscrew the power supply from the back of the case. The power supply is usually held in place by four Phillips screws. Pull the power supply out from inside the case. There is often a small shelf the power supply sits on. Hold on to the power supply as you unscrew it to make sure it doesn't fall.
4). Slide the new power supply into place. Screw it into the case from the back of the computer.
5). Connect all of the drives back to the power supply with the appropriate connectors. Connect the correct connectors to your motherboard as well. Make sure you plug the connectors back in the same direction they were plugged in as there is only one way in which they fit properly.
6). Close the computer case and plug the computer power cable into the power supply and the wall outlet.