H1 for Post Title

I want to update post title from H2 to H1

<div class=”ribbon-more”>
</div>
<h2 class=”posts_title”>XYZ</h2>
<div class=”ribbon-shadow”></div><!– ribbon shadow –>
</div><!– ribbon top –>
<article class=”post_container”>

I want to update this code from H2 to H1 for post title.

I tried everything in Editor, but not able to by default it shows H2 only.

Please help as it is very important for SEO.

Thanks in Advance.