LearnDash: Change the "Sample Lesson" string to something else

snippet.php

<?php
// Change 'Sample Lesson' to exactly how the text appears on your site.
// This could change depending on your use of Custom Labels and translations
// Change 'Free Module' to whatever you want to replace it with

add_filter( 'learndash_course_step_attributes', function( $attributes ) {
	if (
		isset( $attributes[0]['label'] ) &&
		'Sample Lesson' === $attributes[0]['label']
	) {
		$attributes[0]['label'] = 'Free Module';
	}

	return $attributes;
} );