its not that hard, its held in place with 4 screws typically. open, remove screws, remove unit, replace.
there are videos on youtube that will go through the process, watch a few to familiarize yourself with the procedures, more than one video as no one gets it all right.
open the case.
ground yourself.
unplug the power from the PSU and remove all power cables from the motherboard, drives and fans.
insert the new PSU, secure with the 4 screws, plug in the power cable to the PSU, ground yourself, and connect the power cables to the devices.
close case
turn on the computer.
Canned Grounding Rant-
shut down system and remove side panel. with the power cable plugged into the PSU touch a bare unpainted metal area of the case. (my favorite spot is an unpainted screw securing the PSU) once you have grounded yourself you can unplug the computers power cable from the PSU and can touch the system.
if you move your feet, or shuffle in your chair, plug in the cord, reground yourself and unplug again.
end canned rant-