The firmware has an internal version number of 9M6772; whereas, the prior firmware version was 9M6336. The site notes that this isn't the firmware version displayed in the Apple TV Remote settings.
It's unclear what's new but it's likely that the update brings some performance improvements and bug fixes to the remote. Apple has not detailed a way to directly update the firmware. It happens automatically when paired to an Apple TV that is connected to the Internet.
The Siri Remote comes with the purchase of a new Apple TV or you can buy it separately for $59.
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