How do I update an iPod 3G on iOS 3.1.3 to the latest possible version ?

Update it from iTunes on a computer as described at the bottom of this article; devices running an iOS version prior to 5.0 can't be updated straight from the device. It will be updated to the newest iOS release supported by the device’s hardware.

The maximum iOS version for the first generation iPod touch and original iPhone is 3.1.3.

The maximum iOS version for the second generation iPod touch and the iPhone 3G is 4.2.1.

The maximum iOS version for the third generation iPod touch and original iPad is 5.1.1.

If you don't know what device you have, append the last three characters of its serial number to http://www.everymac.com/ultimate-mac-lookup/?search_keywords= and load the page.

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Your iPod can support through the latest version of iOS 5 and can be updated by connecting to iTunes and clicking your iPod on the top right corner (or on the left panel if using iTunes 10 or earlier).

If you have a 3G iPod you need iTunes 10.5 or later on the computer to update.

If your iPod does not have volume buttons on upper left edge and no internal speaker it is a 1G and can't go higher than 3.1.3