Select the heading text
On the Home tab, open the Styles gallery
Click Normal or another non-heading style
To remove heading formatting from multiple headings, select all of them first
Press Ctrl+Shift+N to apply Normal style
If using Heading 1, Heading 2, or similar, change each one to Normal
To remove a heading from the Navigation Pane only, change its style from a heading style to Normal
To remove heading levels from a document outline, open the Styles pane and update the style used by the heading
If needed, use Clear All Formatting on the selected text