To convert a public project to private, right click on the map background, click "Project", then click "Convert project (public/private)".
Alternatively, go to the project settings by clicking on the menu icon in the top left corner of the map. From the project settings popup window, click on "Admin". There you will see "Change privacy". You can change the project from public to private there.
You will only be able to convert a public project to private if you are a paid subscriber to Kumu. To upgrade your account, go to your billing settings, where you can change your plan and add your payment information.
If you want to delete a project, follow this guide.
You can change the privacy of a project through the organization settings page. You can access this by first visiting your dashboard (https://kumu.io/dashboard) and then clicking the "Organizations" tab. From there, click on the "settings" button for your organization and you'll see a list of your projects. Click the project you want to change the privacy of, then click the "edit" button in the window that opens, and then you'll see radio buttons to change from public to private.