It doesn't look like I am able to rename a diagram.
If I right-click a diagram, I only see:
Where is the option to rename a diagram?
If you wish to just change the diagram name hat you see in ER Studio, go to File | DIagram Properties and set the name in the Name field. However, this will not change the actual file name of the .DM1. If you wish to change the actual filename as well, then go to Repository | Diagrams | Change Diagram Filename. You have to do both to keep them in sync...