Apple has released AirPods firmware 3.5.1. AppleInsider.com says it’s probably already been downloaded to a synchronized device.
Installation is automatic when the AirPods are in their charging case, the case is plugged into a charger, and the linked iPhone is in close proximity and connected to Wi-Fi. You can check the installed firmware revision in the “General” settings menu, by selecting “About,” then “AirPods.” AppleInsider says the update contains “miscellaneous minor bug fixes.”