RadImageEditor has a history stack, which records each command that is executed on the image currently loaded in the control. This is convenient, as it allows past actions to be reversed and re-applied whenever needed.
Undo/Redo commands can be executed from the UI.
You can use the methods as well.
radImageEditor1.ImageEditorElement.Undo(); //or radImageEditor1.ImageEditorElement.Redo();
radImageEditor1.ImageEditorElement.Undo() 'or radImageEditor1.ImageEditorElement.Redo()