Understanding Styles in Microsoft Word
Search the page for Manage Styles.

What you are talking about is the "Quick Styles Gallery," I think. The Recommend tab on the Manage Styles dialog lets you set display orders.

Then in the Modify style dialog, your can check the box to add the style to the Quick Styles list.

If in addition to doing outline levels, you are also doing numbering, you really, really, really, need to read the following article and follow its directions, even if it means starting over!


How to create numbered headings or outline numbering in Ribbon Versions of Word by Shauna Kelly