How can I remove the demo mode?
iPhone 7 Plus, iOS 10.0.1
You have to sign into icloud on the Iphone and make sure find my iphone is on.
Go to icloud.com from another computer and it will locate your iphone and press reset.
It will reset your iphone and you will have a brand new Iphone.
It is possible that if you have an iPhone in Demo Mode, that you would be able to restore the device using iTunes. However I would take the device back to where you purchased it from, as a demo model should not have been sold to you in the first place.
Restore your iPhone, iPad, or iPod to factory settings - Apple Support
If you can't update or restore your iPhone, iPad, or iPod touch - Apple Support
Demo mode
