Undo in Liquid Edition has the universally recognized shortcut Ctrl+Z. However the Undo command has some restrictions that can seem confusing to the beginner. The basic rule for the Undo command is that it only affects certain elements on the Timelinenamely, the position, duration, and existence of either a media clip or a transition.
Other elements, such as filters, Timewarp, and any audio alterations are not affected by the Timeline Undo because they have their own integral Undo functions. See Chapters 7 and 9 for full details on how to undo these elements.