If an AirTag is connected with the wrong Apple ID tou can reset it, but it needs to be removed from the other Apple ID first (this is to avoid having your device stolen and used by someone else).
Sometimes this doesn't reset the AirTag itself (maybe it was out of range when it was removed), so a reset is necessary...
Press down on the back (stainless steel with Apple logo) battery cover of your AirTag and rotate it anticlockwise until the cover stops rotating. Remove the cover, then remove the battery.
Re-insert the battery (or add a new one)...
Press down on the battery until you hear a sound. This sound means that the battery is connected.
After hearing the sound, repeat this four more times, removing and replacing the battery, then press down on the battery until you hear a sound. You should hear a sound each time you press on the battery five times. Note that the fifth sound is different from the first four.
The last sound indicates that the AirTag is now ready to pair.
Replace the cover by aligning the three cover tabs with the three slots on the AirTag, then press down on the cover and rotate it clockwise until it stops rotating.
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