We have had legal custody of a child for over 8 yrs and we would like to change her last name. The custody order took place in Alabama and we currently live in TN. How do we do this?

You need to adopt him.

You have to file a petition to adopt her just having guardianship is not enough to warrant a name change. If you are able to adopt then her name will be changed to yours. Why have you had guardianship for so long? Are her parents still alive?