Long documents (for example, manuals) are usually provided with an outline.
To create and edit the outline of a document, Write offers its own outline view with the ribbon command View | group Views | Outline.
In the outline view, you can "designate" normal text as a heading and the outline level of existing headings as higher or lower levels.
Program window in the outline view
Note: Normally, you should not manually change the outline level of headings via the dialog box of the ribbon tab Home | group Paragraph, but you should do this work via the buttons of the outline view as recommended in this example.
If you want to manually change the outline level, go to the ribbon tab Home | group Paragraph and click on the group arrow to open the dialog box and switch to the Paragraph tab. Select the desired level there via the option Outline level.
For more information on working with outlines, see Outlines.