You can move UML Modeling elements in the following ways:
In both cases, you can drag the element onto its destination. Thus, the Projects window can act as a library of easily accessible, reusable objects.
You can drag modeling elements (such as Classes, Packages, and Actors) and some other elements (such as attributes and operations) from one project to another project in the Projects window. You can also nest elements by moving an element under another element in the project tree.
To move elements within the Projects window:
You can drag modeling elements from the Projects window to a diagram that is open in the Diagram editor.
To move onto a diagram an element that is listed in the Projects window: