On that computer, install iTunes, designed to communicate with the iPhone and install the updates. You can download it from Apple's website in the section "iTunes". You will also need the distribution of the firmware version to which you would like to return. You can find her but one of the Western or Russian sites dedicated to Apple products. Here you will find detailed instructions on carrying out rollback to different versions of firmware, and installing updates and the difficulties that may arise in the process.
Connect your iPhone to computer via USB cable and launch iTunes. Wait for the procedure to determine the model of the phone, then this needs to be translated into DFU mode. Simultaneously hold down the "Home" button and lock key and hold for ten seconds. Release the lock button and continue holding the Home button for a few seconds. If you do it right in iTunes, a notification will appear that the device is transferred to the recovery mode. Press release previously keys.
Clamp on computer keyboard special keys depending on your operating system: on Windows it's the Shift button and for Mac Alt. In the appeared window, click "Restore". Specify the path to the downloaded file with the previous version of the mobile platform iOS. After that will automatically start the rollback procedure system, which may take 10 to 20 minutes. At this time do not press any keys on the device, disconnect it from the computer and do not run it a third-party app, otherwise the phone may fail without possibility of recovery. Upon completion of rollback, your iPhone will reboot already with a restored version of the firmware.