Follow the below steps to override default.phtml  in your theme.

Create an default.phtml in : app/design/frontend/Vendor/Themename/Magento_Catalog/templates/product/view

<?php if ($detailedInfoGroup = $block->getGroupSortedChildNames('detailed_info', 'getChildHtml')):?>
    <div class="product info detailed">
        <?php $layout = $block->getLayout(); ?>        
            <div class="product data items" id="accordion" data-mage-init='{"accordion":{"openedState": "opened", "collapsible": true,  "multipleCollapsible": true}}'>
            <?php foreach ($detailedInfoGroup as $name):?>
                <?php
                    $html = $layout->renderElement($name);
                    if (!trim($html)) {
                        continue;
                    }
                    $alias = $layout->getElementAlias($name);
                    $label = $block->getChildData($alias, 'title');
                ?>
                <div class="data item title"
                     aria-labeledby="tab-label-<?= /* @escapeNotVerified */ $alias ?>-title"
                     data-role="collapsible" id="tab-label-<?= /* @escapeNotVerified */ $alias ?>">
                    <a class="data switch"
                       tabindex="-1"
                       data-toggle="switch"
                       href="#<?= /* @escapeNotVerified */ $alias ?>"
                       id="tab-label-<?= /* @escapeNotVerified */ $alias ?>-title">
                        <?= /* @escapeNotVerified */ $label ?>
                    </a>
                </div>
                <div class="data item content" id="<?= /* @escapeNotVerified */ $alias ?>" data-role="content">
                    <?= /* @escapeNotVerified */ $html ?>
                </div>
            <?php endforeach;?>
        </div>
    </div>
<?php endif; ?>


<script type="text/javascript">
        require(['jquery', 'matchMedia', 'accordion'], function($, mediaCheck) {

            var detailsTabs = $('.product.data.items');

            mediaCheck({
                media: '(min-width: 768px)',
                // Switch to Desktop Version
                entry: function () {
                    detailsTabs.accordion({
                        openedState: "opened",
                        collapsible: false                        
                    });
                },
                // Switch to Mobile Version
                exit: function () {
                    detailsTabs.accordion({
                        openedState: "opened",
                        collapsible: true,
                        active: [0,1,2,3,4],
                        multipleCollapsible: true

                    });
                }
            });
        })
</script>