How to change the lesson in selected day if nobody is missing?

If there is no absent teacher or class and you need to edit a particular lesson, e.g. change a classroom (teacher, subject, period), you can use menu "Tools - Change other lesson":

A dialog appears where you can select the lesson you want to change:

You can change the room/teacher/subject as with any other substitution.

The lesson will now appear among the regular substitutions, with yellow color: