How can I change the file system from public to private? I have been using public file system has the upload destination for file field. I want to change public file system into private file system. I cannot change it it showing below error.


  There is data for this field in the database. The field settings can
  no longer be changed.


I cannot select the upload destination. How to change the file system for file field in drupal 8
I have no idea why the UI prevents from doing that. It's not actually anything in the storage, it's just the default for new files.

What you need to do is edit the config file directly, which is named field.field.$entity_type_id.$bundle.$field_name, for example field.field.node.article.field_image.

You can do that using drush cedit, or single export and then import it again from the UI, or do a full config export and import it again after changing that file.