Did you check thru' Nokia Suite also. If the result is same it is because.. Availability of update depends on Product Code / Region. Also, for network locked devices, update is possible only after an approval from the Operator. Hence, even if an update is released, it need not be for your particular device.
Then first you check thru' Nokia Suite. Connect the phone to PC thru' USb. Open Nokia Suite-->Tools-->Software Updates. If it shows an availability, Read and follow instructions carefully.
The Product Code should be written under the Battery. But even if you know that, unless the availability is shown, you can't update.
..and Network locked means the phone is bought thru' your Operator along with the SIM at special rate as a package. If you bought it from Authorised Nokia Dealer and just got the Airtel connection should not put you in restricted catagory as mentioned earlier.
As a last go, you may visit your local Nokia Care and ask whether they can update the device.