very easy imo, I just did it yesterday, and the day before, and before.....
for uninstalling the power supply
first off, unpower your system, disconnect the power cord
open up your case, disconnect all the power wires 1) to the motherboard 2) to the CD rom and DVD 3)hard disks 4) floppy... don't have to do it in sequence thought
unscrew the power supply off your case, there should be 4 screws holding the power supply at the back of the case.
just redo all the steps in opposite when you are installing one.