Base field data is in field_config, and instance data is in field_config_instance. Rather than modifying them manually, though, you can always use the API:

$field = field_info_field('field_name');
$field['key'] = $new_value;

Or for an instance

$instance = field_info_instance('node', 'field_name', 'article');
$instance['key'] = $new_value;

Using the API will clear the appropriate caches, and it’s just a better idea than editing the data manually.

Either way, make sure to backup the database first in case something goes horribly wrong.