< EDIT > if you want to change it in the database (why?), You can do this by changing the value file_directory_pathin the table variable:
UPDATE variable SET file_directory_path = 's:5:"files";';
variable php-. , file_directory_path (s) 5 files. .
, Drupal ( cache cid variables). , , , DELETE FROM cache WHERE cid = 'variables';. </ EDIT >
: Administer > a > http://yourdrupalsite.com/index.php?q=admin/settings/file-system.