Backup your iPhone in iTunes. Create a backup of your iPhone in iTunes so that you can restore from it after the reset. This restore method will not work with a jailbroken iPhone, and will put your device into a boot loop that will require updating to fix. If your iPhone is jailbroken, see the next section instead.
Connect your iPhone to your computer and launch iTunes.
Select your iPhone from the row of buttons at the top of the iTunes window.
Click the "Back Up Now" button with "This Computer" selected. This will create a backup of your iPhone on your computer.