Apple has silently updated the AirPods firmware from version 3.3.1 to 3.5.1.
Read the article to see how to check that your AirPods’ firmware is updated. You won’t notice anything; it’s probably a very small amount of data. But if you want to be sure that you’re up to date, you can check.
Oh, and that version number? Usually we’d be at 1.x.x; it’s surprising that it’s already up to version 3.
Source: Apple Silently Updates AirPods Firmware to 3.5.1 – TidBITS