Overwrite a specific function in child theme

Hello,

what’s the best way to overwrite a specific function in a php file of a parent theme? I mean a specific php file, that isn’t the functions.php file.

In my case, I need to change the default image that appears in slider if there isn’t a featured image. This is the code in my_actived_theme/includes/theme-plugins.php:

// If there is no image, use the default image (available filter: mag_post_image_none).

		if (isset($attachment[0])) {
			$post_image_uri = $attachment[0];
		} elseif ( 'slider' == $size ) {
			$post_image_uri = apply_filters( 'mag_slider_image_none', get_template_directory_uri() . '/images/no-image.jpg' );
		} else {
			$post_image_uri = apply_filters( 'mag_post_image_none', get_template_directory_uri() . '/images/no-image-small.jpg' );
		}

		// If no image, return now
		if ( $post_image_uri == apply_filters( 'mag_slider_image_none', get_template_directory_uri() . '/images/no-image.jpg' ) || apply_filters( 'mag_post_image_none', get_template_directory_uri() . '/images/no-image-small.jpg' ) )
			return $post_image_uri;

		// If MU/MS install, we need to dig a little deeper and link through the blogs.dir.
		if ('IS_MU') {
			$imageParts = explode('/files/', $post_image_uri);
			if (isset($imageParts[1])) {
				$post_image_uri = '/blogs.dir/' . $blog_id . '/files/' . $imageParts[1];
			}
		}

		return $post_image_uri;
	}
endif;

So how can I edit this code? (in my example, I need to change the image url).

I hope my English is clear. :/

Thanks in advance! ??