This filter was added in Caldera Forms 1.8.6
When a Caldera Forms file field is NOT set to upload the file to the media library, you can use the caldera_forms_private_upload_directory filter to change the directory the file is uploaded to.
If a field does upload to the media library, you should use the caldera_forms_upload_directory filter instead.
How To Change The Upload Directory For Caldera Forms Private File Upload Fields
Where Does This Code Go?
When using WordPress hooks to customize Caldera Forms or other plugins you should not modify the plugin files, or you will lose your changes when you update the plugin. Instead you should create a small plugin to hold the custom code. It's easy, learn how here.
Technically you can add the custom code to your theme's functions.php, but then you will not be able to change your theme and keep these customizations.