IPhone 3G just keeps on asking for an update

I purchased an iPhone 3G with model number a1303. I synced it with my computer for the first time. iTunes said it needed an update so I run the update then disconnect it from syncing. After few days, when I need to upload music to my iPhone, I synced it again with iTunes and I have been asked again for an update. I disconnect then reconnect it again but the same thing happened. What should I do next? Help please!
