On the diagram, model elements can be deleted by selecting them and then either:
selecting Delete on Home tab of the ribbon or
pressing Del key
Deleting an organization item in the diagram deletes only the organization row from this diagram. To delete the organization item completely, open the organization item dialog and click the Delete button or delete the organization item from the navigator window.
Deleting an element with the "Allow elements with no instances" behavior option enabled deletes only the element from this diagram, not the model.
Deleting a process step automatically deletes all connected groups and all connectors that will not be connected to any process steps after the deletion. Deleting a process step, flow or a group also deletes all measure values connected to it.