1. Prepare some version from archive wordpress.org

2. In the root deirectory of the site, delete “wp-admin” and “wp-includes” directories.

(FFFTP is convenient)

3. ftp all the directories and files except “wp-content” folder.

(You should not delete .htaccess naturally.)

(You shoud not delete wp-config.php naturally.)

(wp-config.php would not be replaced, because native wp module should not have it.)