What are you using to show the article? just normal single article menu item? i.e. "page content" particle? If so you would need to create a joomla html override for the article - see joomla docs for that. But... there should only ever be one h1 tag on a page. That is why the h1 tag is associated with the "page title" field on the menu item "display options" tab. Article titles are normally h2 because there can be more than one shown on a page.
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