You can change the room of a lesson by clicking on the <Allocate/Delete this room> button or by right-click into the timetable and selecting the menu item of the same name.
How to swap rooms
If the room allocation dialogue window also shows the occupied rooms, you can use them depending on the selected option (room clash, room swap etc.).
If the option "Allocate additional room" is activated, you can add to the existing room another room.