If you decide to sell your iPhone or want to know how to reset iphone to factory settings please follow these steps:
How to Reset iPhone to factory settings
This is the official link to Apple website to restore your phone to factory settings :
Before resetting it to factory settings make sure your all the contacts and other useful data is backed up on iCloud.
Step 1 – Start your iPhone and then tap on Settings
Step 2 – Now scroll down and tap on iCloud
Step 3 – Now make sure your contacts , calendar or any other data which you need to backup must be switched on (Green Button) to synch with iCloud .
I hope it helps you to restore your phone to factory settings , if you need any assistant please call us 0189 2544199 and we will help you.