Instructions
1
Connect your iPhone to your computer, using the USB docking cable. Open iTunes if it does not open automatically.
2
Select your iPhone's name under "Devices" on the left side of the iTunes window. The Summary page appears.
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3
Click "Restore" in the center of the main page. Choose to back up your iPhone, then agree to the terms and conditions and confirm that you wish to restore. The restore may take several minutes. Leave your iPhone connected at all times. When the restore is finished, a screen offering setup options appears in the main iTunes window.
4
Choose to set up your iPhone as a new phone, or select a backup from the drop-down menu to use for the setup. Follow the prompts to complete the setup. If you set up your iPhone as a new phone, you will have to name it and adjust your syncing preferences as you did when you first got it. If you set up from a backup, you do not need to make any further adjustments.
5
Leave your iPhone plugged in to your computer until the sync is completed. If you disconnect your iPhone before iTunes tells you it is OK to disconnect, you will damage your phone's OS again and will have to perform another restore.
Read more: How to Reload an OS on an iPhone | eHow.com http://www.ehow.com/how_8574963_reload-os-iphone.html#ixzz20EHGbHJF