Most desktop computers use three-wire cooling fans to cool the motherboard and other parts. These fans, while small and relatively cheap, provide one of the only safety measures to keep a computer from overheating. The operating system of the computer keeps track of the internal temperature in the computer's case and adjusts the fan's speed as needed. If a fan begins to fail, or more cooling is needed after a component upgrade, installation of a new fan may be required.
Locate the fan dimensions if replacing the fan. They should be listed under the computer specifications in the computer's owner manual. There are only a few common sizes, usually ranging from 72 to 120mm. Turn off the computer and remove the side case screws from the back of the computer. Most computers have only two Phillips-type screws holding the side of the case on.
Remove the side panel of the computer tower. Locate the three-pin fan connector socket on the motherboard by following the wires coming out of the fan. Holding the fan's connecting end, gently pull the connector out of the socket. Be careful not to bend the pins coming out of the motherboard, as they will break easily if bent. Remove the four screws holding the fan to the case by unscrewing them from outside of the case. Most fans are screwed into a ventilated part on the back of the computer.
Plug the replacement fan into the motherboard's three-pin fan socket. The three pin connections are designed to only fit the correct way. If the fan cord is twisted, do not try to force the connection; instead, rotate the end to align the connection properly. Screw the new fan into the computer case and tighten each screw so that the fan does not rattle. Replace the side panel of the computer and screw the case back together. Power the computer on and watch or feel for the fan to turn on when the computer starts to ensure the new fan is operating.
If upgrading computer parts, such as a graphics card, a stronger fan may be useful. Check the computer specifications for the fan dimensions, and look for a fan that fits and has a higher output rating.