When
you open a UML diagram, it is displayed in the diagram editor. You can have
multiple diagrams open at the same time and use the tabs at the top of the
editor to navigate between the them.
To open a diagram, in the Project Explorer view,
navigate to one in either the Models folder or the Diagrams folder and double-click
it.
Tip: You can also open UML
diagrams in the Model Editor view on the Details page.