post title as h1 instead of h2

Hello there,

I want my post title to be a h1 heading, instead of the default h2. I’m aware that h1 is used by the custom page title, which I don’t apply in blog posts. So in terms of SEO I’m currently wasting the important h1 heading because it’s not being used in posts.
On the other hand I do use custom page titles on pages.

Is there a way to change the headings of all existing posts from h2 to h1? Which file do I have to edit?

Thx.