Scroll down and select Device Preferences → About → System update .
Before updating, disconnect all USB peripherals and ensure your power supply is stable. A power outage during firmware installation is the only way to permanently brick your Mi Box S. Have you experienced a different issue after the update? Power cycle the device (unplug for 60 seconds) before attempting any advanced troubleshooting. Xiaomi Mi Box S Firmware Update
Click Download and wait. The Mi Box S will continue working in the background during the download. Scroll down and select Device Preferences → About