Copy index.php to root. Edit the file and add subdirectory

require( dirname( __FILE__ ) . '/directory/wp-blog-header.php' );

Edit .htaccess file

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /directory/
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /directory/index.php [L]
</IfModule>
# END WordPress