Undo ONLY works if UNDO is enabled. See Preferences options.
Select Undo on the quick access menu to revert changes that have been made to your simulation. All recent changes will be displayed in the UNDO menu and you can select the point you want to revert to.
The UNDO button on the quick access toolbar undoes the first (top) item in the UNDO menu. The shortcut key for this is CTRL+Z. The pop up hint on the undo button shows the same text as the first item in the menu.
The simulation time will change to zero (the time the clock starts) when you UNDO changes unless you change preferences to switch off this default setting.