Undoing and restoring operations

You can move between your changes in the timetable by menu View - Undo/Redo:

Please note that this only affects the positions of the cards. It is not possible to Undo changes in the contracts.

It is a good idea to use File - Save as... to save the timetable on disk under different name. That will allow you to store a semi-finished version of the timetable, to which you can return in the future.