This feature, change copyright, is available only in premium version.
Sorry but our development strategy not allow edit the copyright for free themes.
Copyright is a unique public Credit link allowed by WordPress community. It is a unique public link what can be attached to theme developed by us. It is unique link visible by sample users, and what represent us and our right like author on the theme.
I think you do not mind and understand why I can not help you to remove this link.