How to update my ipod by computer

If I understand you correctly, you want to install the Viber app to your iPod Touch correct? Why not just downloaded it directly from the App Store app on your iPod Touch over Wi-Fi?

Note that it requires iOS 4.3 or later. So if you have a 1G or 2G iPod Touch, you won't be able to install the app onto the device as the 1G can only be updated to iOS 3.1.3 and the 2G iPod Touch to iOS 4.2.1.

B-rock