To change the HTML output as part of the calculator you can override the Stamp Duty Calculator template into your own them and make the necessary changes. This also means you won't lose the changes should you update in future.
The steps to override the template are:
- Create a folder called propertyhive in your theme.
- Copy the template(s) from %YOUR_PLUGIN_DIR%/propertyhive-stamp-duty-calculator/templates/stamp-duty-calculator.php to %YOUR_THEME_DIR%/propertyhive/stamp-duty-calculator.php