It took quite a lot of effort to get the old stereo out. You need the U shaped release keys (I found some on ebay for 99p that did the trick) which you push into the holes on the sides of the current stereo until they click, then bend them outwards and try and pull the whole thing out. I think that some models just pull out as it is, but mine was screwed on at the back... we didn't realise this until we got the stereo out though! Myself and my housemate managed to break it off with a lot of jiggling and brute force, it didn't seem to damage the rest of the stereo. Once that bit's out, it's a case of just unplugging wires from the back of one unit and into the new one. It's fairly easy to see where everything fits once it's in front of you - I'm definitely no expert and managed fine! Then it's a case of just slotting the new stereo in the gap, and you're done. I haven't yet got around to trying to wire in the steering wheel controls - that may be a job for over Christmas.
I hope that's helpful =]