"i cannot use my ipodtouch 2G? "
Why not? What happened? Any info about your issue?
"and also i cannot donwload or update my ipodtouch 2G to the latest firmware for bug and fix? "
Like all computers/tvs/cars/dvd players, etc old hardware cannot run the latest software forever. It simply cannot handle it. The second gen was released over 4 years ago - very old in tech terms.
"so how can i update ipodtouch 5.1.1 2G? "
You cannot. 4.2.1 is the end of the line