How do I update my iPod 3G to iOS 5?

I'll make an educated guess and say that it is this model,

"Following the introduction of the third generation iPod Touch, Apple also continued offering the second generation 8 GB version, upgraded to the same iPhone OS version as the new third generation devices."

http://en.wikipedia.org/wiki/IPod_Touch

You actually have a 2G iPod touch. Those can only go to 4.2.1