Word provides built-in styles for several different levels of headings and subheads—Heading 1, Heading 2, and so on. The styles define the font family, size, color, and more. While you can create your own styles and use those as headings, or adjust the formatting of headings look on the fly, you can also change the default styles if you want.
How to Customize a Heading Style
On the “Home” tab in Word, you’ll find some built-in styles in the “Styles” group, including the Heading 1 and Heading 2 styles.
You can right-click either of those heading styles and then select “Modify” to get started customizing them.
But Word also includes a bunch of other heading levels—nine in all. To see them, click the little arrow at the bottom right of the “Styles” group.
By default, the Styles window that opens shows only recommended styles, which is the same group Word shows on the Ribbon. To change that so you can see all the styles, hit the “Options” button.
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Source : How to Change the Default Heading Styles in Microsoft Word
