Upgrade error

I am trying to upgrade to the lastest version and have replaced all files except
wp-content,wp-images,wp-config.php, .htacess file

and get this error
Fatal error: Call to undefined function: require_wp_db() in /home/homefind/public_html/blog/wp-settings.php on line 206

for this site:
homefinders.com/blog

Please advise what to do