Changing data in \responsive\core\includes\functions.php

hi ,

i need to change a word (string) in this file

\responsive\core\includes\functions.php

note : responsive is my theme .

i want to change it from “posted by” to “Author”
i changed it and worked fine but is that correct ?should i change it directly from this location , i think it is better to override it in my child theme folder , but when i try to copy the “core” folder , error happened , white page displayed , so how can i override this file ?