To convert a public project to private, right click on the map, click "Project", then click "Convert project (public/private)".
Alternatively, go to the Project Menu 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.
*Note: 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 dashboard settings to change your plan and add your payment information.