1. First of all, if you didn't back up in iTunes before upgrading, you need to back up in iCloud, because in iTunes, the backup of a higher version system cannot be restored to a lower version system.
2. Open the following website: /wetools/index.php? r = io rom/index/mid/35
Select the corresponding iOS 10 firmware to download.
3. After the download is completed, enter the recovery mode according to the following operations.
① Confirm that the iPhone is connected to iTunes, and press and hold the lock button until the Apple logo appears.
② Then press and hold the Home button, and the screen lock button does not need to be released.
③ When the screen turns black, release the lock button and continue to press and hold the Home button (the Home button behind the iPhone7 is virtual, so press the Home button instead of the volume button).
4. Then the following interface will appear on iTunes.
5. Hold down shift, click Restore iPhone, and then select the firmware you just downloaded.
6. Wait for the brushing to complete, and then restore the backup from iCloud.