A lesson's date can be changed in several ways:
1. Drag the lesson to a new date and drop it. Note this does not work on mobile devices.
2. Edit the lesson and click on the lesson's date. Then select a new date and hit update. This is useful to change the lesson to another week, or for any change on a mobile device.
3. Bump the incomplete lesson earlier or later using the cell menu. Note - this impacts other lessons.