I read on news website , they say Apple just release firmware 3.1.3 after Dev team unlocked firmware 3.1.2 . Is that true ? Can I update 3.1.3 on my itouch ?
1.) We don't talk about the dev-team stuff. Read the board rules.
2.) Yes, you can simply update to 3.1.3 through iTunes (but lose the jailbreak).
3.) iTunes will find the update for your iPod touch automatically. And yes, it's called the "iPod touch". There's no such product as an "itouch" AFAIK.