Apple on Wednesday released a revised version of the 1.0.276 firmware designed for AirTags, a revision to the original update that was released earlier this month.
As noted by MacRumors, the new AirTags 1.0.276 firmware has a build number of 1A287b, while the old firmware had a build number of 1A276d. The original 1.0.276 firmware added anti-stalking security enhancements, and this new version likely just offers a refinement of those features.
AirTag firmware updates install automatically when an AirTag is in range of your iPhone. The process is similar to how Apple updates the firmware of its AirPods lineup.
Checking your AirTag firmware version is a bit roundabout, but here goes.
If your AirTag has not yet been updated, be patient. Just make sure your AirTag is in range of your iPhone and it should soon automatically update.