All operations in the Timeline window can be undone and redone.
To undo an operation
Click the Undo button to undo an operation.
Click the list button of the Undo button to display a list of operations that you have performed and select one of them to undo.
To redo an undone operation
Click the Redo button to cancel an undo and put the operation back into effect.
Click the list button of the Redo button to display a list of operations that you have undone and select one of them to redo.