The Redo option is found on the Quick Access toolbar, alternatively you can use Ctrl+Y.

The Redo functionality is controlled by the Redo button visibility setting under System Configuration in Diagrams options.

When the Redo Button Visibility option is set to True, then the Redo button is visible in the Quick Access Toolbar. The button is hidden when the setting value is set to False.

If you enter Ctrl+Y when the option is set to False, a warning message will be displayed.