The Table of Contents.hhc file is auto generated by the toc.xml and htmlhelp_stylesheet.xsl files. 
You really can edit it if you want but I suggest you use the stylesheet to auto generate it instead.

If you want to regenerate the table of contents file you can do so with
the command "msxsl toc.xml htmlhelp_stylesheet.xsl" if you are using msxsl.exe.