One of the support team members in an Apple Store once suggested: you need to leave AirPods connected to the Mac inserted into open case, which is plugged and charging for about 30 minutes to upgrade the firmware. Though in my experience there is definitely a random factor in play for such an upgrade. Moreover I have an impression that even Apple Store employees sometimes have very vague idea how Apple products interplay with any other Apple product excepting the iPhone. Two times I had hardware issue with 1st and 2nd gen Airpods they were very confused that I don't use those with iPhone but with a Mac and Android only.