The stories created within each project have a default privacy setting. You can change the default privacy setting on the Project Preferences page of each project you want to change the setting for.
You must be a Project Manager or an Organisation Admin to access the Project Preferences page.
This does not work retroactively. It does not change the Story Privacy Level of existing stories within the project, it simply changes the default story privacy setting for stories created from this point onwards.
Steps
Access the Project Preferences page of the project you want to change the default story privacy setting for
Scroll down to the Default Story Privacy section
Change the privacy level to the level you want to use going forwards
Scroll to the bottom of the page and press Save