Example of how to customize unfiltered string content using PHP instruction str_replace

post-content-nav.php

add_filter( 'wp_link_pages_args', 'change_link_pages_args' );
/**
 * Change the Link Pages Arguments - a demonstration of how to change
 * "unfiltered" (meaning not passed through apply_filters())
 * string data using PHP instruction str_replace().
 *
 * @since 1.0.0
 *
 * @param array $args
 *
 * @return array
 */
function change_link_pages_args( array $args ) {
	$args['before'] = str_replace( 'Pages:', 'Read More:', $args['before'] );

	return $args;
}