Relatively easy. You will need some Kebab skewers to remove the old unit. You stick them in the holes on the side then push them out to the side and pull. It will pop out. Unplug the four plugs in the back, connect them to the adapter/patch lead and then connect it to your new stereo. Check it works before you push it back into the dash!
If you need to buy a lead that will allow you to keep your steering wheel controls I have one for sale for £25. I also have a black stereo surround that fills the gap around the outside.