The airbag light can be reset by connecting an ODBII reader. However the light is on because of a problem with the airbag system. The system needs to be checked to insure that the airbag works properly.
To reset your airbag light, in a Nissan Altima, you should turn the car on. From there, as soon as the airbag light stops flashing, turn the car off instantly. Repeat this three times to reset the airbag light.