You have to do the work yourself on this one by modifying your style definitions.
If your lowest - level style is Heading 3, then change the formatting of Heading 4 to be the same as that of Heading 3. Move up the level until you have Heading 2 now looking like Heading 1. Then use Heading 1.


If these are numbered, you should be following the directions in this web page to set the numbering: How to create numbered headings or outline numbering in Ribbon Versions of Word by Shauna Kelly