Changing a fan on a power supply unit.

S0litaire

Unless your 100% confident working around PSU's then don't touch it!

I've knows a friend or 2 end up on the other side of the room after touching the wrong part. (one guy could not use his arm for a good few hours! another ended up in A+E with concussion, cracked his head off of the concrete floor!)

If you still want to do it then:

Make sure it's totally unplugged from the mains and PC for a good few hours. You can also try and discharge it by unplugging it from the mains and holding the PC power button in for 20 seconds or so, that helps discharge the internal capacitors before removing it.

Make sure you are earthed using a wrist strap or have and earthing mat to work on!
Make sure you've got something soft behind you!
And someone to drive you to A+E lol

(ok last 2 are a joke but you never know....)