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This should be done before doing any kind of PC repair work which requires opening a computer. If you neglect this step, you may damage your computer. This may include keyboard, mouse, network cable and speakers. It will be connected to almost every component inside the computer case and will look something like this: Unscrew the mounting screws on the back of the case which hold the power supply in its housing.  Place the screws in a handy location. This is usually a simple process, but if there is little room in your PC, it may be necessary to remove other components to get the power supply out. If you don't feel comfortable removing other components, replace the mounting screws and enlist the help of a PC expert before continuing. Do not try to yank a power supply out by force.

Summary:
Familiarize yourself with proper ESD procedures. Disconnect all external connectors (including the power cord) from the machine. Identify the power supply unit. Remove the case cover. Gently remove the old power supply from the case.