DK, the generate TOC sub-tool is the greatest thing going, but as posted, it depends on your having set up a hierarchy of headings using <h1> and so on, down to <h4> in some instances. The tool then allows you to include all or or only some of them. And note that this tool creates only the NCX or virtual TOC, not the html or actual TOC in the front of the book.
For the latter, you can use Create Html Table of Contents, but of course you must use the same hierarchical headings. Personally, I prefer my own layout, so I write this in advance. Sigil changes the links as I break the book into individual chapters and sections.
If you have used paragraph styles for your headings, just do a search & replace. And ensure that you have the heading styles defined in your style sheet.
Tools > Table of Contents IMHO is one of Sigil's greatest features.
There's an (outdated) Sigil user's manual that I can no longer seem to locate online. This appears to be it in PDF form:
http://peterloveland.com/development...2.0-Manual.pdf
And here is the downloadable epub:
https://github.com/Sigil-Ebook/Sigil...ide_0_7_2.epub