Need to update my first generation IPod touch

I have an IPod touch model no. A1213. I don't even have an apps icon on my main menu. I read I can upgrade my firmware to get the apps icon. I went to ITunes, My Devices, and it said I can update to 1.1.5. I want to update to 3.1.3 which is the latest version (so I've read) that I can get. I can't find this update anywhere on here. Is it still available somewhere? Need a link please!
Thanks!