I've Joomla site that was upgraded to latest version. Latest version is within /new/ folder. How do i instruct apache with the .htaccess file to redirect all demands to /new/ folder rather to public_html?

The easiest way is always to alter the virtual host file and indicate the brand new path.

But when you shouldn't alter the root path, you should use mod_rewrite to do this, using something such as this inside your .htaccess file:

 RewriteEngine on
 RewriteRule ^(.*)$ new/$1 [L]

It might grab everything following the domain title, and append it after 'new/' Make certain you will find the mod_rewrite module enabled.

You'll find a lot of how you can set rules in mod_rewrite here

This really is helpfull when yuo convey more redirects inside your .htaccess

Options +FollowSymLinks
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /

RewriteRule ^(.*)$ new/$1 [L,R=301]

</IfModule>