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'] = $new_value; field_update_field($field);
Or for an instance
$instance = field_info_instance('node', 'field_name', 'article'); $instance['key'] = $new_value; field_update_instance($instance);
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.