First off the warranty has expired so I don't care about voiding it.
Is it kinda a hard thing to mess up?
There is a sticker on the PSU that says 'internal parts are not serviceable.' However I think that is just to scare people off. If I could slide it out I don't think it would be too hard to replace the fan.
However I am afraid if I mess up then I will have cost myself $100 or so dollars in having to buy a new one.
If there is no warranty, go ahead and open it up. If you have any mechanical skill it should be obvious what to do. Some are soldered, others use a 2 or 3 prong clip. You might need to leave the original fan connection in place and splice your wires in. Should use standard sizes and parts, though finding a 135mm fan is probably more difficult then the standard 120mm.
Air flow won't be as good for one. Won't learn as much or see whats inside the PSU is another. The PSU doesn't work already, so as long as you don't apply to much force to anything you can't really make it worse.
You may follow these steps (at your own risk):
1. disconnect the computer from the power outlet (<-very important obviously)
2. press the computer's power on/off button a few times to discharge the PSU's caps (<-very important)
3. disconnect the PSU from the other system components and remove it from the computer case
4. remove the appropriate screws from the PSU casing and open the cover
Here you should see how the fan is connected to the PSU board. Typically, it's a simple 2-pin or 3-pin(less common) power connector(soldered wires are pretty rare these days) and all you have to do is disconnect it (it'll have small tabs holding it in so a little force is required) and replace the fan. If your setup is different or if you need more detailed instructions, then post some pics of the PSU's interior so we can tell you how to proceed.
If you're trying to replace the original fan with a different model that may complicate things.
Edit: I found some interior pics of your PSU. I didn't notice you had it listed in your sig.
Logan the Huge pointed out what's important to not electrocute yourself. As long as you do that, and are careful not to damage the PSU, you should be fine. While you can just splice the wires for the fan, make sure you crimp the wires properly and leave no wire exposed. I'd also cover them in electrical tape and make sure they are put away neatly so as not to melt and expose the wire by being in contact with a heat sink or something. If you're good with a soldering iron, then of course just solder it.
When you're done, you may want to use a PSU tester to make sure it still works as expected rather than testing it out on your rig. Just a suggestion.