Unless ur a retard (which is actually pretty common these days) these things are pretty safe.
Best way to discharge a psu is put it in bios, then just turn it off at the wall. Even if you dont, by the time u crack the case and the psu open they usually discharge on their own in ~30sec. Unless u leave it plugged in. In which case you probably should die so you dont breed and pass on your retard genes.
That will suck all the power out. Opening will void your warranty. Ive opened nearly all psu's ive had for one reason or other, including changing to quieter fans.