You have to edit the file wp-content/themes/royal/framework/woo.php the code <?php _e('SIZING GUIDE', ETHEME_DOMAIN); ?>.
Also change the title in wp-content/themes/royal/woocommerce/single-product/up-sells.php.
You will find the code: etheme_create_flex_slider($args, __('Our offers', ETHEME_DOMAIN), false, true);
and change “Our offers” title to anything you want.
We recommend you to make this customization in Child Theme.