Edit default header content through Theme Editor

I have never used this theme before, and while I usually can navigate the theme editor files on my own, I would rather not break this site. What I want to be able to do is change one of the two buttons on the site hero so that I can enable for the button to have an OnClick function to copy whatever I have in the to the clipboard.

I just need to know the location of where I can edit this. For reference, I am looking to do this exact thing: https://www.w3schools.com/howto/howto_js_copy_clipboard.asp

But with the server address. Any help appreciated on where I can edit the:
<?php mesmerize_print_header_content(); ?>

The page I need help with: [log in to see the link]