Disconnect the power cables from the motherboard and from the drives and any fans that it is connected directly to it.
Remove the screws on the back of the case that is holding the computer's power supply in place. Hold the PSU so it doesn't fall once the screws have been remove.
Carefully remove the power supply from the computer case.
Place your new power supply in the same location of the old one. Make sure the location of screw holes line up. Install the screws to secure it in place.
Connect the PSU wires to the motherboard, drives, fans etc.