how do i connect to a used airtag ?

You can reset the AirTag.

i think you have to remove the battery and reinstall it.

If you google it there’s an apple support article that explains it

if found the article:

How to reset your AirTag - Apple Support

How did you come across this "used" AirTag? Did you buy it from someone or just found it? If the former, you will need the original owner to remove this tag from their Apple ID. Unless they do so, you will not be able to use this tag as it will still be associated to them.

If you found it, you will have the same issue.