If iPhone backed up the original ios system before upgrading ios 10, you can directly restore the backup to the original ios system. However, if you don't make a backup before upgrading the system, you can't restore to the original system. After the ios system is upgraded, the demotion entrance of the previous system will be closed.

1. Apple will regularly release software updates to solve emerging security problems and provide brand-new functions; This update is usually provided to all supported devices at the same time. Users will see iOS update notifications on devices and iTunes, and updates will be sent wirelessly to encourage users to apply the latest security fixes as soon as possible.

2. The above startup process helps to ensure that only the code signed by Apple can be installed on the device. In order to prevent devices from being degraded to earlier versions that lack the latest security updates, iOS adopts a process called "system software authorization". If the device can be degraded, once the attacker takes control of the device, he will install an earlier version of iOS, and use the unrepaired vulnerabilities in the old version to destroy it.

3. For devices equipped with A7 or higher A-series processors, the Secure Enclave coprocessor will also use system software authorization to ensure the integrity of the software and prevent degraded installation.