Best to disconnect battery for at least 10 minutes before working on airbag.
Airbag is fired by a small, but powerful, explosive device. It has a capacitor to store electricity for a while after power is lost, so that it is still available without power. This will gradually reduce to zero in 10 minutes or so.
It has been known for airbags to fire when disconnected if there is still power in the capacitor. This is why the Fire Service are campaigning for cars to have two earthing points that they can use to deactivate teh bags as they can fire when they cut roofs off. The airbag fully inflates in three tenths of a second and will do you harm if you are in the way.
Disconnect battery.
Wait ten minutes.
Remove airbag carefully.
Place well out of the way, face up, so you don't tread on it or trip over it.
Once replaced, keep everyone out of its way when reconnecting battery.
Unwanted deployments are rare, so don't worry, just don't take risks.