Install iTunes Backup from different iPhone with iOS 10.2
First of all, iTunes backups don't contain firmware, so they can't upgrade or downgrade your device. They only contain data like messages, contacts and application documents.

You won't be able to install iOS 10.2, as it is no longer being "signed" by Apple. Only firmwares being signed by Apple can be installed on devices - as such, only iOS 10.2.1 can be installed. You can check which firmwares are currently being signed by Apple at https://api.ineal.me/tss/status.