The examples in this guide have been performed on a test document can be downloaded here.
This section, accessible by clicking on the icon that represents an index, will allow you to navigate through the document for possible headers or for their modification.
LibreOffice - Ubuntu
In the top of the window you can find a list box where you can select a header search or a summary of the most common ones in the document. You can also directly edit the field by typing a regular expression that will be the key to search for the possible headings to search for. In the latter case, in order to start the search will have to click on the button next to it.
If you make a click on a line (as in the first example) will be searched and shown that occurrence in the document. At this point you can modify the header level (1 to 4) or to delete it by selecting a different style, but not the header.
Very useful is the ability to select multiple lines together by applying the same heading style.