When
resetting Airtag, you are clearing its old association, making it possible to be set up with a new Apple ID or device. This is to physically open the AirTag, replace and re-replace the battery five times by pressing until you can hear the pattern of the chimes, and close the cover again. After that, you can take it and bring it near to your iPhone or iPad and then go through the pairing process to get it working like a fresh one.