Back up your music, applications, photos and other stuff
First of all you need to make back up of all of your stuff present in your iPhone. You need to back up your photos, contacts, music, videos and all other stuff which you have stored in your iPhone.
It is better to make back up for your data in external storage devices, your computer hard disk or any other storage drive before upgrading your iPhone. This is a precautionary step as you might lose some of your files during firmware upgrading. Apple removes some files accidentally when upgrading its firmware.
Restore to its original settings
If you have installed any unauthorized software, games or any application then you should restore your iPhone settings to default. In case you have downloaded applications or games from Installous or used hacking methods to jailbreak your iPhone then the firmware upgrading may face problems. Thus it is necessary for you to restore your iPhone settings to original.
You can restore your settings by simply connecting your iPhone to your computer with the USB cable and launch iTunes. Click ‘restore’ button from its menu and wait for a while. Your iPhone memory will automatically be cleaned as this step will reset your iPhone.
Connect iPhone to your windows and launch iTunes
Launch iTunes after plugging your USB cable to the computer. If there is an upgrade available, you iTunes will notify and ask you. It is recommended to upgrade your software whenever an update is available.
Allow iTunes to upgrade
Leave your iTunes to upgrade your firm. This will take some time.
Remember no to use your computer or iPhone during this process. You won’t be able to receive any calls or messages during this span.
Unplug your iPhone
After installation is complete, unplug your iPhone. Confirm your upgrading by tapping ‘Settings’ then ‘General’ and then ‘About’ where you can see version number.
Remember to remove your phone after installation is fully complete and iTunes notify that it is safe to disconnect.