If I want "Manual" to link to a different place, what needs to be modified?

Thank you.

It depends on where you want to move it and how it should be integrated :)

Would you retain the layout of the forum? Should it be transferred within the directory structure of the forum, or outside?

Put your question please just a little more closely, so users here can respond more accurately.

In setup_options you find
$manualIndex='index.php?action=manual'; // or 'manual.html' for mod_rewrite

And there, with the correct entry, ... you could, for example, reach your goal. There are of course other ways ... but just depends on your exact requirements :)

Adding to the above: link to the Manual is put under templates/main_header.html -> {$l_menu[4]} tag. You could replace it to whatever else, or if you need something more specific, let us know.