Turn the steering wheel 180 degrees and disconnect the battery for 15 minutes. Push a flat head screwdriver through the small hole on the underneath (now the top) of the wheel with the edge of it at 12 and 6 so that it catches the metal wire release mechanism inside - you will feel it flex. Pull on the airbag unit with a bit of force, it takes a lot, but is a hefty piece of equipment so don't be afraid of it.
Once it's off, take a picture of how everything sits. You'll need a Torx attachment to take off the bolt, don't worry too much as the wheel itself has a TDC mark on it so won't go squiffy.
The airbag connectors have a strip on top of them with a serial number printed, you need to prise the strip up in order to release the lugs in the socket - do not force them out of the socket until the strip has been raised!
Fitting the new wheel is the reverse of the above.