If you're using a UIS Drupal template, you can change the heading level using the Format dropdown menu in the Primary or Secondary Body editor.
Think of your webpage like an outline.
The page title uses the first heading level (<h1>), so all below sections should use heading level 2 (<h2>).
Any subsections within your h2-level sections should use h3, any subsections within your h3-level sections should use h4, and so on (until h6).
But I like the style of h3 better than h2!
If you want the first section on your page to look like an h3 or h4, add class="h3" or class="h4" to your h2 tag.
You can do this for any header on the page! For example, the following header is an h3 that looks like an h5.