Restore your iOS device in iTunes on Mac
You can use iTunes to reinstall the software on your device and restore it to its default settings. You might need to do this if you’re having problems with the device (for example, if it freezes or crashes frequently) of if you’re getting ready to sell or give away your device.
Important: Restoring iOS software on iPod touch, iPhone or iPad deletes all your information and settings. However, iTunes backs up your device before restoring it, so you can use the backup to restore your device to how it was when you last used it.
Connect your device to your computer.
In the iTunes app
on your Mac, click the Device button near the top left of the iTunes window.
Click Summary.
Click Restore, then follow the onscreen instructions.
Restoring iOS software on your iOS device deletes all your information and settings, but doesn’t erase it. Deleted information can’t be accessed through the device’s controls, but it remains on your device. To find out more about erasing all content and settings, see the Apple Support article What to do before you sell, give away or trade in your iPhone, iPad or iPod touch.
If you’ve enabled Find My iPhone, Activation Lock is on. You need to turn off Activation Lock before the new owner can activate iPhone under their own account. For information about turning off Activation Lock, see the topic in iCloud Help Activation Lock in Find My iPhone or the Apple Support article Find My iPhone Activation Lock.