Easy, if you ignore the fact you're working on a very expensive computer
My mid-2011 iMac was making a buzzing sound; quiet if the LCD screen was off (screensaver) but louder and annoying when the machine was active.
I finally lost patience, and a brief search on the web indicated that the PSU would be the culprit.
It's normally left switched on 24/7; I powered it off last week and it wouldn't switch back on, which reinforced the theory that the PSU was on its way out. The machine powered back up after being left for 24 hours, but by that point I'd already ordered the replacement PSU from iFixit, and my mind was made up.
I've opened up my iMac once before, to install an SSD alongside the factory HDD, so this wasn't completely terra incognita for me. In the years since installing the SSD I'd lost my suction cups for removing the glass screen cover; I'm happy to report that a $2.50 pair of suction cup hooks from Home Depot (OOK-brand 5lb hooks, if I'm allowed to mention a specific product) did the job well - but take care, of course, because that glass is fragile.
Following iFixit's guide made identification of the cables easy, and removal of the screen was pretty quick.
I took the opportunity to give the inside of the machine a good dusting - gentle use of a compressed air canister whilst using the house vacuum overhead to catch the dust.
Everything went back together quickly and easily, although the screen data cable is fiddly and very delicate.
I am typing this review on the iMac, so - job done!
Do this in a clean, dust-free environment if possible.
Once you remove the glass panel, the LCD screen attracts dust like honey attracts ants!
Have somewhere safe, stable, and out-of-the-way to set the glass panel and the LCD unit while they're removed.
Have a compressed air duster handy (ideally, the invertable sort that doesn't spray out any propellant liquid), to gently blow any dust off the LCD and the inside of the glass panel as you re-assemble.
DO NOT TOUCH the face of the LCD panel. Without its protective glass screen, it's soft, very delicate, and fingerprint marks are difficult (even impossible) to remove. It is NOT the same as a typical laptop LCD panel, which incorporates a protective layer.
Take the opportunity to blow any dust out of the inside of the iMac; the two lower fans and the finned heatsinks are particular dust-traps.
When removing the old PSU, and mindful of the iFixit guide's dire warnings about high-voltage capacitors, I wore a stout rubber yard glove; I'm sure it made me look daft (like a green-fingered version of Michael Jackson?), and was possibly an excess of precaution, but I felt less nervous. I suspect that an alternative would be to leave the iMac for a day or two after disconnecting it from the mains supply, to give the caps chance to discharge.
The two small cables for the screen (v-sync and display power) are keyed, but the connectors aren't very stout. It might be worthwhile to use some white-out or a small sticky label to mark the orientation of the connector BEFORE unplugging it, to ease the task of re-assembly. Whilst holding the LCD unit up, and trying to plug the cables in with only one hand, is not the best time to have doubts about which way round the cables go. A head-mounted flashlight (or cellphone flashlight) helps here.
There are several strong magnets mounted to the chassis, along each side of the screen, that retain the glass screen cover. When removing/reinstalling the LCD fixing screws, these magnets want to grab the screws off your screwdriver. As long as you have tolerably small fingers, you can guide the screws into their holes without much difficulty.