Sets user field to a specific value.

Update_user

$results = db_select('users', 'u')
  ->fields('u', array('uid', 'created'))
  ->condition('uid', 0, '<>')
  ->condition('uid', 1, '<>')
  ->range(0,300)
  ->orderBy('created')
  ->execute()->fetchAll();

foreach($results as $result) {
$user = user_load($result->uid);
$user->field_profile_recieve_alerts['und'][0]['value'] = 1;
$user->field_profile_receive_discussion['und'][0]['value'] = 1;
user_save($user);
}