When your AirPods are charging and within range of your iOS device or Mac running macOS, any available software updates will be downloaded and installed automatically. You can’t force AirPods to update, and you can’t update them manually from a device that isn’t an Apple product.
Apple has updated its support website for AirPods firmware upgrades to include information for users who aren’t directly connected to an Apple device. If you don’t have access to an Apple device, you may arrange a firmware update appointment at an Apple Retail Store or with an Apple Authorized Service Provider.
If your AirPods are stuck on an older firmware version, or if you use AirPods with an Android or Windows smartphone, this advice may help. Even though there probably aren’t that many, Apple felt compelled to add this to its support post today.
Even when not related to this particular edge case, updating the firmware on AirPods is a nuisance. The AirPods cannot be forced to update, and you cannot manually install updates either. Apple also does not provide detailed release notes when updating the firmware of the AirPods.