Q: hello I would like to update my I pod touch version 3.1.2, how I have to do?

Tap Settings > General > Software Update

Or connect your iPod to a computer and update using iTunes.

More details >  iOS: How to update your iPhone, iPad, or iPod touch

Carolyn Samit, the Tap Settings > General > Software Update requires iOS 5 which the OP says they do not have.

gzucca

To update

The Settings>General>Software Update comes with iOS 5 and later.

Connect the iPod to your computer and update via iTunes as far as your iPod model allows

iOS: How to update your iPhone, iPad, or iPod touch

A 1G iPod can go to iOS 2.2 via iTunes and iOS 3.1.3 via

Purchasing iOS 3.1 Software Update for iPod touch (1st generation)       

https://buy.itunes.apple.com/WebObjects/MZFinance.woa/wa/touchLegacyLandingPage

A 2G to 4.2.1. Requires iTunes version 10.X. If a Mac it requires OSX 10.5.8 or later.

A 3G to 5.1.1  Requires iTunes version 10.5 or later

Identifying iPod models