change in form-checkout.php by using code in functions.php

By injecting CSS (not optimal):

function remove_message_text() {
      echo '<style type="text/css">#order_review_heading { display: none; } </style>'; // Remove original text "Your Order"
      }
      add_action( 'woocommerce_checkout_before_order_review', 'remove_message_text');