Not clear from your query but seems you see there is an update available for your device on the internet ( may be official Nokia site) but when you check thru' the phone it says No updates available..then you may first check thru' Nokia Suite . Follow the procedure as given here .
If the result is the same, the reason is that updates are made available depending 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.