You have to edit 3 files

H1 in Header

Depending on which template you are using the logo Alt tag is normally set as a H1 tag. This is not good for SEO. There should be only 1 H1 tag on any particular page so having on in the header of every page is not good. To remove this tag  edit the following:

\app\design\frontend\default\YOUR THEME FOLDER\template\page\html\header.phtml

Line: 30

here you can edit the title. i usually will make it <strong> or <h3> – up to you.

Category Headings

/app/design/frontend/default/YOUR THEME FOLDER/template/catalog/category/view.phtml

there should be around 3 to 4 pairs of “<H>” tags, just change them from whatever heading they currently are to H1’s.

Product Page Headings – Make the Product name an H1

/app/design/frontend/default/default/template/catalog/product/view.phtml

change the current H tag to <H1>

If you want to change the style and size of the H1 tag, you can use CSS. Also if your custom theme folder doesn’t have those files, edit the ones in the Default template folder instead.