How can I change book icons to folder icons in TOC in CHM?

Standard chm template creates book icons.

If you want to change to folder

you need to specify it in HHC file by this tag:

<param name="ImageType" value="Folder">

To do it, you need to modify your template. Go to Templates folder and open your template in a text editor. Find the line containing this text:

<param name="Window Styles" value="0x800025">

add the following line after it:

<param name="ImageType" value="Folder">

Save the file. Then just compile your documentation using new template.