Apple is rolling out a new version of its AirTags firmware (version number of 1.0.291 and a build number of 1A291f, up from 1A291e). While there is no way to force your AirTag to update its firmware, there is a way to see if it has been updated to the latest version.
The most recent build number was only available to 25% of users, this build number removes the deployment limit completely, meaning the firmware should be available to all users.
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.
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.