The latest firmware version for the Apple AirPods is 6.3.2. The AirPods was last updated March 25, 2019.. The previous firmware version before this was 3.7.2.
To check if your AirPods is up-to-date, go to Settings > General > About > AirPods.
To update your AirPods, simply put the AirPods into the charging case, connect the charging case to a power source, and make sure your iPhone is nearby. The AirPods will automatically update.