To login into magento admin, using only ftp access is a little tricky. Through FTP open the class Mage_Admin_Model_User located at
app\code\core\Mage\Admin\Model\User.php
Next find the
$this->loadByUsername($username);
You need to add the line return true; after this i.e
$this->loadByUsername($username);
return true;
And that’s it, now you login in admin using any password.