The content actual path is : Theme/view/frontend/templates/html/notices.phtml . directly changing content in core files is not recommended way. instead of that you can override those template file.
create layout file for override global notices
app/code/Vendor/Module/view/frontend/layout/default.xml
<?xml version="1.0"?>
<!--
/**
* Copyright © Magento, Inc. All rights reserved.
* See COPYING.txt for license details.
*/
-->
<page xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:View/Layout/etc/page_configuration.xsd">
<body>
<referenceContainer name="after.body.start">
<block class="Magento\Theme\Block\Html\Notices" name="global_notices" template="Vendor_Module::html/notices.phtml"/>
</referenceContainer>
</body>
</page>
Create a template file to override content app/code/Vendor/Module/view/frontend/templates/html/notices.phtml
<?php
/**
* Copyright © Magento, Inc. All rights reserved.
* See COPYING.txt for license details.
*/
// @codingStandardsIgnoreFile
?>
<?php
/**
* @var $block \Magento\Theme\Block\Html\Notices
*/
?>
<?php if ($block->displayNoscriptNotice()): ?>
<noscript>
<div class="message global noscript">
<div class="content">
<p>
<strong><?= /* @escapeNotVerified */ __('JavaScript seems to be disabled in your browser.') ?></strong>
<span><?= /* @escapeNotVerified */ __('For the best experience on our site, be sure to turn on Javascript in your browser.') ?></span>
</p>
</div>
</div>
</noscript>
<?php endif; ?>
<?php if ($block->displayNoLocalStorageNotice()): ?>
<div class="notice global site local_storage" style="display: none;">
<div class="content">
<p>
<strong><?= /* @escapeNotVerified */ __('Local Storage seems to be disabled in your browser.') ?></strong><br />
<?= /* @escapeNotVerified */ __('For the best experience on our site, be sure to turn on Local Storage in your browser.') ?>
</p>
</div>
</div>
<script>
require(['jquery'], function(jQuery){
// <![CDATA[
(function($) {
var test = 'test';
try {
localStorage.setItem(test, test);
localStorage.removeItem(test);
} catch(e) {
$(".notice.global.site.local_storage").show();
}
})(jQuery);
// ]]>
});
</script>
<?php endif; ?>
<?php if ($block->displayDemoNotice()): ?>
<div class="message global demo">
<div class="content">
<p><?= /* @escapeNotVerified */ __('This is demo store. No orders will be fulfilled.') ?></p>
</div>
</div>
<?php endif; ?>
Change your content whatever you want in "message global demo"