How to change "read-more" text in the child theme ?

I have responsive theme installed and also have made child of that theme activated.

I would like to change the text of the “read more” in the blog-archives and author page.

I found that it can be done in the function-extras.php file in the in core/include folder and have managed to do so.

But I would not want that change to disappear with updation of the theme.

So, how can I do the change in file of child theme, please guide.