how can i update my ipod tuch 2 gn

Update from what?

What do you have now?

It can only go to 4.2.1

Connect to iTunes, click on your device in the device list and then cklick on "Check for Updates".

The latest version for your device will be shown or downloaded, which for the iPod 2G would be iOS 4.2.1

Since most of the apps available today in the Appstore need at least iOS4.3, you might not find many apps that can be used on your device.

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

iPod touch: Which iOS 4 software features does my iPod touch support?