Detach
When you click the Project tab to open the backstage menu, a sortable list of attached projects is displayed (when you Attach a project you make it visible to the application).
To detach a project and remove it from the list, click on the ellipsis and select Detach from the drop-down menu:
Multi-Select to Detach
When you click the Project tab (or select an option from the Home | Project menu) to open a project via the Backstage menu, you can also multi-select the projects you want to detach in the Open Project list:
You will be prompted to confirm or cancel each detachment (or all detachments):