I know Joomla by default always puts the Article Title in H2 tags. If your goal is to change all H2 Tags to H1 when creating articles you can use an extension such as the Aimy H1 Heading
extension to accomplish that rather than creating a template override. It will search a page to see if an H1 tag already exists. If it cannot find any H1 tags on a page it picks the first H2 and turns it into an H1.