You can position a task as a priority directly on the task by clicking on the "Arrow" icon at the top left of the task.
You can also switch the "Priority Task" cursor between active or inactive in the task detail panel. To do this, click on the task and go to the "What" tab. The slider turns blue when it is active.
Good practice: There can only be 2 priority tasks per task list to prevent all tasks from being prioritized.