Registration for Warranty for Apple AirPod

That should happen automatically when purchased at an authorized reseller. Check here:

Check Your Service and Support Coverage by clicking here --> https://checkcoverage.apple.com/

If it didn't:

Click on this link to read how to 'Update your proof of purchase' --> https://support.apple.com/HT202703

To update your information contact Apple Support by phone. Tell the advisor that you need help with your Apple Limited Warranty or AppleCare plan expiration date.