Flashing an electronic device refers to changing or updating the firmware on the device. Firmware, simply put, is the software that runs on the hardware of the device. Reflashing your iPhone refers to restoring the original version, or an older version of the iPhone's firmware. You may want to do this if you are experiencing problems with the current firmware on your iPhone.
Hold down the "Power" and the "Home" button on your iPhone until it turns off.
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Release the "Power" button when the phone turns off but keep holding the "Home" button. Plug the iPhone into your computer using the USB cable.
Wait until you see the "Connect to iTunes" prompt. Make sure to continue holding the "Home" button until this appears.
Release the "Home" button when iTunes recognizes your iPhone.
Click on the iPhone icon in the left pane of iTunes. Then click the "Summary" tab.
Click the "Restore" button and iTunes will restore the original firmware on your iPhone.