Override template file details.phtml in your custom theme.
Path: app/design/frontend/[vendor]/[your-theme]/Magento_Catalog/templates/product/view/details.phtml
& replace all the code with the below code.
<?php if ($detailedInfoGroup = $block->getGroupSortedChildNames('detailed_info', 'getChildHtml')) :?>
<div class="product info detailed">
<?php $layout = $block->getLayout(); ?>
<div>
<?php foreach ($detailedInfoGroup as $name) :?>
<?php
$html = $layout->renderElement($name);
if (!trim($html)) {
continue;
}
$alias = $layout->getElementAlias($name);
$label = $block->getChildData($alias, 'title');
?>
<div>
<h3><?= $block->escapeHtmlAttr($alias) ?></h3>
</div>
<div>
<?= /* @noEscape */ $html ?>
</div>
<hr>
<?php endforeach;?>
</div>
</div>
<?php endif; ?>
Just for your info: By the way, Magento provides the best way to show description & specification in tabs. if you still want to change then you can replace that file code.
Updated
To change the title, Go to your catalog_product_view.xml (also override this file) & search for the below code.
<argument translate="true" name="title" xsi:type="string">More Information</argument>
& replace it with.
<argument translate="true" name="title" xsi:type="string">Specification</argument>