This is a known concern. Having multiple h1 was detrimental to SEO in the past. But, that is not the case now. X was built using HTML5 and having 2 h1 tags is structurally valid because they're part of different areas. Also, in modern SEO in combination with HTML5, having multiple h1 will not have an impact on your ranking. X was built with SEO in mind so it's structurally optimized. You should focus on quality content instead.
If you still wish to continue changing the tag, regretfully, there is currently no option for this so you'll need to override the template files that contain h1 tags. Please see our Customization guide for more details. Just note that it requires web development knowledge. If you have no experience modifying templates, I'd recommend that you consult with a third party WordPress developer as this has the potential to break your site.
The visually hidden h1 is in _brand.php file located in x\framework\legacy\cranium\headers\views\global. Copy that file to this folder in your child theme x\framework\views\global. In the copied file, change the h1 tag in this line:
<?php echo '<h1 class="visually-hidden">' . $site_name . '</h1>'; ?>
Other files that has an h1 tag are _landmark-header.php located in \framework\legacy\cranium\headers\views\integrity and wp-comments.php located in \framework\views\integrity.