Thanks for asking.
Please note that the customisation you are looking for will require changes in theme files which falls outside the scope of support we can offer. However to help you get started I can point you in right direction.
First, as this will require changes in theme files I suggest you to setup child theme. You can use following resources to download and setup child theme.
Download Child theme from following source: https://theme.co/apex/child-themes
Please take a look at following article to setup child theme:
After that you can copy _index.php from
/wp-content/themes/x/framework/views/ethos/ and paste under
/wp-content/themes/x-child/framework/views/ethos/. Please note that you need to create the child folders in the same order. Next you can add the headings manually after line number 12.
To learn more about HTML heading tags, please take a look at following resources.
Heading Tag: https://www.w3schools.com/htmL/html_headings.asp
You can also take a look at following videos on YouTube by Matt Cutts on Heading and SEO: