You can Rename a diagram from the Document Manager by hovering over the diagram name, then clicking to edit.
Once you click it will allow you to start typing a new name. If you do not want to change the name, click off the name and it will revert back to the original. Click off the name once done changing and it will save the new name.
You can also Rename a diagram by opening the diagram up in the Editor (double-click), then go to File->Rename.
A new dialog box will open where you can enter the new diagram name, then click “Rename” to save your changes.