Backing up your device ensures you have a copy of your information for when a factory reset is completed in the coming steps.
Follow the steps in the link below to complete a back-up of your device.
IMPORTANT: Do not use iCloud to back-up your data, but instead use iTunes.
Restoring your device’s software requires to you to perform a factory reset, this is where a new version of the phone’s software is downloaded. Your data will be wiped, although if you have performed a back-up prior to resetting the device, you will be able to restore it in the next step.
Follow the link below for steps on how to complete a factory reset on your device.
Follow the steps in the link for steps on how to restore information on your device.