Use undo/redo
When editing a model it can be easy (as is often the case with diagram editors) to make an addition or change you immediately wish you hadn't made. Returning the diagram to how it was before the change manually can be a nuisance involving several mouse manipulations and/or key clicks. Any change can be undone by using the undo button on the toolbar or pressing Ctrl z. You can undo changes incrementally by repeated undo operations and if you change your mind there is a redo button.
Undo/redo can make your diagram editing more fluent, saving time and patience when you are up against a deadline.