Move the lesson
If you do not want to have a lesson to be taught, but move it to another day, then right-click on the line with this lesson and from context menu select "Move". In the next window the program shows you all suitable possibilities for this teacher and this class. These possitions are marked with green.
You can also move the lesson to previous/next week. Simply click on "<<Week" or "Week >>".
Replace the lesson
Swap the lesons