You can modify the heading hierarchy or assign a level in the hierarchy to a custom paragraph style. You can also add chapter and section numbering to heading paragraph styles. By default, the “Heading 1” paragraph style is at the top of the outline hierarchy.

To Add Automatic Numbering to a Heading Style

1. Choose Tools – Chapter Numbering, and then click the Numbering tab.
2. In the Paragraph Style box, select the heading style that you want to add chapter numbers to.
3. In the Numbers box, select the numbering style that you want to use, and then click OK.

To Remove Automatic Chapter Numbering From a Heading Paragraph

1. Click at the beginning of the text in the heading paragraph, after the number.
2. Press the Backspace key to delete the number.

To Use a Custom Paragraph Style as a Heading

1. Choose Tools – Chapter Numbering, and then click the Numbering tab.
2. Select the custom style in the Paragraph Style box.
3. Click the heading level that you want to assign to the custom paragraph style in the Level list.
4. Click OK.

Source: Outline Numbering – LibreOffice Help