The firmware upgrade process should not take more than 2 minutes. If even after 3 minutes the reboot device message is not seen, the device can be safely restarted manually. This is an issue that occurs rarely with some combinations of browsers and operating systems
Even if the device is powered off in the middle of an update, when the device restarts it will wait for firmware upgrade. So the device can always be updated, even if something goes wrong during the update process.
Cymatic Audio Support