The software can suggest you some moves that are possible and may help you.
The following picture shows that software noticed that you do not need to substitute the 2nd lesson. Instead of assigning new teacher, you can move the lesson from 6th period to 2nd because 6th lesson's teacher has free time:
You can ignore this suggestion and assign any other teacher to substitute the 2nd lesson.
If you decide to accept this suggestion, simply click on the small arrow on the 5th period and select "Suggested moves":
A following dialog appears. As you can see the software suggests to cancel 2nd lesson and move 5th to 2nd.
Press ok to accept this suggestion. Then on the main screen you can see the result:
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