The passenger airbag switch is on the passenger end of the dashboard. You can see it when the front passenger door is open.

Turning the airbag off
Pull the switch outwards and turn it to OFF. The passenger airbag is disabled and the OFF status indicator lights up.
Turning the airbag on
Turn the switch from OFF to ON. The airbag is enabled and the ON status indicator lights up. It is now safe for an adult to use the front passenger seat.

Warning:Always switch the passenger airbag back ON when an adult sits in the front passenger seat. The driver is responsible for making sure the airbag setting is right for the passenger.
Warning:If you must fit a rear-facing child seat in the front passenger seat, the passenger airbag MUST be switched off. Otherwise a young child could be seriously injured or killed in an accident.