Even if it looks as if the HTML Help editor lets you directly edit the help file, this is not the case. The HTML Help editor is the user interface which lets you work in a user friendly WYSIWYG manner on the files that together form a help project. You will need to recompile the the help project to get a new help file with all changes you did to the source files included. Recompilation can be started from the HTML Help editor menu.
Note that recompilation is in fact a two step process:
If the first step is skipped, for example by starting compilation direcly on a .hhp file, the content tree and index will not reflect the latest changes.