This article provides step-by-step instructions for replacing AirTag batteries and answers important questions about charging AirTag batteries and other topics.
How to remove and replace the battery in your AirTag — Apple Support
If the battery is low or empty, you can replace the battery in an AirTag by following these steps:
Place the AirTag with the stainless steel back facing up.
Press the back with your fingers and turn counterclockwise until the valve can no longer turn.