How can I edit index.php manually in html???
C5 doesn't work like a static html website, you'll need to edit the files you find in the *active* theme folder for your site.
You can find out the active theme in the admin end, and the files will probably be located here:
I'd recommend reading up on the basics in the documentation before getting too involved with this side of things as you need to have at least a superficial understanding of how C5 works before getting your hands dirty theming.
Hope that helps,