normaly there are some torx screws on the back the steering wheel either 2 or 3. undo these and the bag will be free to remove, just unclip the power supply plug from the bag caution thou its said that you should disconect the battery for about 15-20 minutes before you do this, and also disconnect before plugging the power supply plug back in. The only problem i can see is that you may wipe the immobiliser codes if the battery is left disconnect for a time scale.
if its anything like the 01-05 model, its a hole in the lower half of the back of the steering wheel, insert a screwdriver in there and push to unhook a spring clip that locks the airbag in position. once unhooked, the airbag will slip upwards slightly. then pull up from the top rear edge. it should unhook easily once unlocked.
always ensure you have disconnected the battery before disconnectig airbags etc..
make sure you have side lights on when reconnecting the battery in order to prevent power surges on the ecu/dashboard.