* Connect your device to a computer and open iTunes
* While your device is connected, force it to restart
* Don't release the buttons when you see the Apple logo
* When you get the option to restore or update, choose Update
Or other method similar to first one but it is easy way
* If your screen is black or frozen, you might need to force restart your device. A force restart won't erase the content on your device. You can force restart your device even if the screen is black or the buttons aren't responding. Follow these steps:
* Press and hold both the Sleep/Wake and Volume Down buttons for at least ten seconds, until you see the Apple logo.
* On an iPod touch: Press and hold both the Sleep/Wake and Home buttons for at least ten seconds, until you see the Apple logo.