It is mega-easy!
Of course, turn off your computer and unplug it from the wall :b
Just unplug all of the power cables from your motherboard, vid card, drives, and fans, etc. Make sure to remember where they go.
Then, just unscrew the PSU from the back of the case, and slide it out.
Then slide the new one in and screw in the screws.
Then, connect the power connectors to all of the devices, and double check to see that everything is plugged in.
Now you can plug the PSU into the wall, and turn on the computer.