If the phone says "no update available", then you are already running the latest available SW version for your phone. SW update availability depends on several things, such as country/operator approvals. See.
It has been now close to 6 months when the 3.51 SW update originally started rolling out and I'm sorry to say, but if your phone has not received the update yet, it's quite possible that it also won't be getting it. We try to publish the updates as widely and quickly to for all the users as possible, but if some country or operator has not approved the new SW version, phones running that specific SW variant will not also get the update.