woocommerce_product_upsells_products_heading.php

<?php
/**
 * Hook: woocommerce_product_upsells_products_heading.
 * Modify the text in the upsells section title.
 */
function plugin_republic_product_upsells_products_heading( $text ) {
  // Modify the $text here
  return $text;
}
add_filter( 'woocommerce_product_upsells_products_heading', 'plugin_republic_product_upsells_products_heading', 10, 1 );