Im not sure about tercel cover, I believe the COVER is attached permanently to the airbag. So there should be a screw or a retaining clip holding the bag in place. Its usualy behind the steering wheel, on the steering wheel.
I wouldnt try to replace the airbag, they are a bit tricky, i researched some more online,

if u dont install it right they might detonate in ur face, which is preaty scary
U can try to repair cover with some plastic. U might need to replace the Trigger for the airbag, wherever ur car got hit, which deployed the airbag. If u dont, ur new airbag might detonate
1. Disconnect the negative cable from the battery and wait about 5 mins.
2. At the bottom of the Steering wheel, pry up the retaining tabs and remove the access panel from underside of the air bag. Now you can get at the air bag connectors.
3. Remove the air bag connector and unplug it (it's tricky so make sure you remove it properly, don't force it)
4. Remove the right and left steering wheel covers (don't break the tabs)
5. Remove the 2 (4?) bolts that holds the air bag.
6. Air bag should come out pretty easy now, don't force it...
Heres some info i found online. Be careful!