From The Document Foundation Wiki
How do I modify the style/format of a title?
The default style/format of Heading 1, Heading 2, or Default style is easy to modify.
To update it:
- display the Styles and Formatting sidebar using ▸ (F11);
- select the text which already has the format you want;
- in the Styles and Formatting window, click the button on the top right New Style from Selection; and
- select Update Style.