Apple has released a new version of the 1.0.276 firmware for AirTags, which is an update to the previous version released in June.
The build number of the new AirTags 1.0.276 firmware is 1A287b, whereas the prior firmware was 1A276d. Anti-stalking security upgrades were added to the 1.0.276 firmware in June, and this new version is likely merely a refinement of those capabilities.
Because the firmware version has not changed, the Find My app will not be able to distinguish between the new and old firmware, nor will it be obvious when the firmware of the AirTags has been updated.
Because an AirTag update is done over-the-air using a linked iPhone, there is no way to force it. You can place your AirTag in range of your iPhone to ensure the update happens, but you’ll have to wait for the firmware to be rolled out to your smartphone.