The easiest way to change the date or time of a lesson is to drag & drop it to a different time slot. However, the time will be snapped to the nearest value of your diary slot.
Another way is to double click on the lesson and change the time directly in the time box.
The diary will notify you if the time overlaps with another lesson.
If you right click on the scroll button, a preset time list appears.
See also: How to use scroll control