Here is how to change document root for a domain in cPanel using .htaccess file:
RewriteEngine on
RewriteCond %{HTTP_HOST} ^domain.com$ [NC,OR]
RewriteCond %{HTTP_HOST} ^www.domain.com$
RewriteCond %{REQUEST_URI} !folder/
RewriteRule (.*) /folder/$1 [L]*where domain.com is your domain name and folder is the new document root
For more .htaccess snippets check My .htaccess Cheatsheet
