If you make a change in the Composition Editor and then decide that you should have left things as they were, select Undo from the Edit pull-down menu to restore your visual part to its previous state. You can undo as many operations as you want, all the way back to when you opened the Composition Editor for the current part. As soon as you close the Composition Editor for your part you lose any ability to undo or redo any changes made.

If you undo an operation and then decide that you did the right thing in the first place, select Redo from the Edit pull-down menu. Redo will restore the view to the state it was before the last Undo.