Enqueue WordPress

functions.php

<?php

// All scripts to load here
function load_scripts() {

  // Register script
  wp_register_script('scrollup', get_template_directory_uri() . '/path/to/scripts/jquery.scrollUp.min.js', array('jquery'), '2.3.3', true );

  // Enqueue it
  wp_enqueue_script('scrollup');
}

// Add action to load scripts enqued in load_scripts() function
add_action('wp_enqueue_scripts', 'load_scripts');