Check out the process to change the Apple Airtag batteries as follows below -
1. Check the metal cover behind the Airtag device. Push down and move anti-clockwise.
2. Whenever it stops rotating, pull up to remove the cover or battery.
3. Put the new battery with a positive facing up.
4. The airtag beeps if you place the new battery correctly.
5. Place the cover back as it is, checking the correct slots.
6. Now move the cover clockwise to lock the cell.
Read More - https://allsavvy.net/airtag-battery-life