Simply add the code below to your wp-config.php file:
define('FTP_HOST', 'ftp.yoursite.com');
define('FTP_USER', 'Your_FTP_Username');
define('FTP_PASS', 'Your_FTP_password');
define('FTP_SSL', true); // If you can use a SSL connection set this to true
Thanks to Jesse for this tip!
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WordPress tip: easy upgrades using FTP