Remove h1 from site title

Duena is built with HTML5. In HTML5 you can have multiple H1s on a page.

But is a good thing to do from seo perspective?

If you can find decent evidence to support the claim that multiple h1s is bad then that’s fine. I, personally, haven’t seen any.
http://www.youtube.com/watch?v=GIn5qJKU8VM

And if i want, how can i remove it?

1. Create a Child Theme: http://codex.wordpress.org/Child_Themes
2. Copy the header.php file from the theme’s folder and paste it into your Child Theme directory.
3. Edit the header.php file that’s now in your Child Theme
4. Replace the “h1” with “h2” there.