Open demo5.gpn. On Monday, teacher "New" is absent. Newton would have had 4 periods to teach on this day. The first period is to be cancelled. Go to the substitution line with your mouse and click on the <Cancelled> button. Clicking on this button a second time turns it back into an uncovered substitution.
Enter "-" for the substitution teacher to cancel a lesson. Clicking on <Del.>, it changes back into an uncovered substitution.
When you check the "Uncovered substitutions" box, only those substitution cases are shown which still need editing.