1. Connect to your server via FTP(Client such as FileZilla or Hosting File Manager) and download your wp-config.php file
2. Open the file in an HTML or code editor
3. Add the following to the bottom of the file beginning on a new line.
   
   PHP

   define('FTP_USER', 'username'); define('FTP_PASS', 'password'); define('FTP_HOST', 'ftp.example.org:21');

   1 2 3

   define('FTP_USER', 'username'); define('FTP_PASS', 'password'); define('FTP_HOST', 'ftp.example.org:21');

4. Make sure there are no blank lines at the end of the file, then save it.
5. Ensure the file is saved in the same format, ie. do not save it as wp-config.php.txt or else
6. Back on your server, rename the existing file to wp-config-old.php and upload the edited version. If anything goes wrong you can delete the edited copy and restore the original to wp-config.php

Reference:

http://codex.wordpress.org/Editing_wp-config.php#FTP.2FSSH_Constants