If you already jailbreak your iPhone and want to reverse the process then you can do it easily. The only thing require is iTunes, no extra software or tool is required. The process uses iTunes restore feature. In between the process you can backup all your files and all the settings of your phone, contacts, apps can be restored later. The unjailbreak process applies to all iOS devices including iPod touch, iPad and iPhone. Here is a method to do it:
1. If you don’t have iTunes installed in your computer then download it from here.
2. Connect your iPhone/iPod touch to your computer via USB cable and open iTunes on your computer.
3. In the left sidebar click on your device.
4. Go to Summary tab, click on “Restore” button.
5. Select “Yes” when the message will appear which asks whether you want to restore from backup. It works even if you are restoring from the backup which you made after jailbreak your phone.
6. It will take some time to reverse the process. Once it will complete you’ll get your old non jailbreak phone.
I am not sure whether returning from jailbreak phone to non jailbreak will restore the warranty or not. If you know the answer then you can mention it in the comments.