Apple periodically releases updates to the version of iOS used on the iPhone, iPad, and iPod Touch. This document explains how to identify which version of iOS firmware a device is using.
Go to the home screen of your iOS device and click on Settings.
Click on General.
Click on About.
Note the number after Version; this is the version of iOS installed on the device. For example, the device pictured is running iOS firmware 4.2.1.