Edit the H1 tag for "site-title" H3

I note that the "Site-title" is in H1 tag. With the title of the page or article, it has been two H1 tags on the same page! (annoyance for SEO) I would put the "Site-title" H3 tag. What is the file to edit and in which directory he found? After research, I can not find it! (The site is being created in localhost) Pending read you, cordially Olaf