Apple has provided some versions of the firmware used by AirTag to process it data. The purpose of these, is to continually provide security against stalking and make improvements in the anti-stalking features for the AirTag trackers.
The AirTag cannot update its firmware alone, unlike with other apple devices like iPhones and iPads you only need a WiFi connection and available storage to update. With the AirTag, it can only be updated when in close proximity to the paired or registered device.
You just have to make sure that your AirTag is in the range of or proximity to your iPhone, and it would automatically update itself.
If you want to check the current firmware version of your AirTag, you just need to open the Find My application on your iPhone, then choose the Items tab in the bottom navigation bar. From there, you choose your AirTag from the list of items by tapping on the name of your AirTag. With just this simple step you can see the serial number and firmware version of your AirTag.