why can't I update my ipod touch 3.1.3 to 4.2.1

Check the model of your iPod:

http://support.apple.com/kb/HT1353

If it's a first-generation iPod touch, iOS 3.1.3 is the last version of iOS your iPod will run. If you have a second-generation iPod touch, post back with more details than "can't update", including the operating system your comptuer is running and the version of iTunes, and we can probably help further.

Regards.

The original iPod touch's hardware doesn't support iOS versions newer than 3.1.3.

(78633)

Are you sure it is a 2G iPod?

What model number is listed on the back?

iPod touch (2nd generation) is model A1288

You can also easily ell the difference between a 1G and 2G by the shape of the black antenna cover on the back.

Identifying iPod models

It's a first generation iPod touch.

(78640)

From other reports, that model is a first-generation unit. Look at the back of the iPod and in the text below the engraving, look for the model number. The 2nd generation iPod touch is model A1288.

Googling shows you have a 1G iPod. Those can only go to 3.1.3