how to override file in inc folder?

I have installed WEN Corporate theme and created a child theme.
This theme includes a widget for Recent Posts. It automatically shows the Featured Image below the post title. But only the title is linked to the post. I would like to also link the image to the post.
The only place I can find a reference to <?php the_post_thumbnail( ‘post-thumb’ ); ?> is in a file called theme-functions.php, which is located in the theme’s inc folder.
I have modified the file and placed it in a inc folder in my child theme, but nothing changed.
I have looked around and it seems like I am supposed to call the file from my child’s functions.php file. But the file does not seem to be being called by the parent’s functions.php – at least I don’t see it there. How is it working at all if it’s not being called from somewhere?
Regardless of that, how can I call it? I don’t really understand php too much, so I rely on copy and pasting from the original – but I have nothing to copy so I have no idea what to do!
Please, can you inform me of what text to put in my child’s functions.php file so it will call my modified version?
Thank you!