I supposed that your WordPress installation is under /site folder. If it's true, you can do it:
If you already has changed the URL, on General panel, to http://mypage.com (without /site), you just need to copy the index.php
(under site/
) to the root folder.
After copy the index.php file, edit it, adding site/
to the path to wp-blog-header.php
:
require( dirname( __FILE__ ) . '/site/wp-blog-header.php' );
The detailed steps of this process are on "Using a pre-existing subdirectory install " at WordPress docs: http://codex.wordpress.org/Giving_WordPress_Its_Own_Directory