Not difficult at all. Drop your exhaust, jack up the bike from the underside of the engine until the swingarm is completely slack, pull your shock and reservoir, remove the fan (remembering how you pulled it out, because it can be a pain to put it back in), and pull the O2 with a crows foot O2 socket.
Edit: Okay, that sounds way more complicated than it actually is. I promise it's super straightforward. it's a little tedious, but it's not a difficult process.