To rearrange a course's units or nest units below another unit (to become sub-units), go to the "Dashboard" (1) tab in the course's page. From there, click on the "Edit mode" (2) link (at the top right) or the switch next to it (to make it go green) and start editing the course.
In the "Course Content" section hold your mouse on a unit (3) to drag and drop it in a new position or below another unit (as a sub-unit). Drag the unit over its new position and drop it on the dashed area (4) that appears between existing units.
To nest it as a sub-unit, drop the unit over its future parent and slightly to the right (5).
When the sub-unit is nested, an angle icon appears right before the parent unit's name (6) to allow for the sub-unit list to collapse or expand.
All edits to the units' order are auto-saved. If you drop a unit in the wrong place, just drag and drop it again into the preferred position. Please note that when you edit units from the lessons library, your edits might affect other courses that use the same lessons. Also, don't forget to switch the "Edit mode" off before proceeding to edit a unit's content.
Note: When the "Edit mode" is on (the switch is green), you can also click on a title to change the course's or a unit's name. To save your changes, just click out of the name field you just edited.