The new HomePod software will be installed automatically on the HomePod after you update to iOS 11.4, but you can also manually update. The update introduces AirPlay 2 and HomePod stereo streaming.
AirPlay 2 is an updated AirPlay protocol that's simpler, more advanced, and more robust, enabling multi-room audio capabilities. With AirPlay 2, you can play music in any room to any room, move music from one room to another room, or play the same song in multiple rooms using an iOS device through an app or Control Center, the Apple TV, the HomePod, or Siri voice commands on any of the aforementioned devices.
Stereo pairing uses an Apple-designed wireless peer-to-peer direct link to allow the HomePods to communicate with one another and play music in sync. The two speakers will act as one when paired, and just one will respond to incoming Siri requests.
Alongside all of todays other updates Apple has released a minor update to iTunes which is focused on performance improvements.