Hi,
You can't re-order tasks in Jira epic by dragging and dropping them from one position to another. Drag and drop feature is not available in epic direct view.
But there is a way how reprioritize your tasks in epic. What you can do, is change the order of tasks by clicking on
Order by. You can re-order your tasks by Creation date, Key, Priority, Status or Assignee:
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Order your tasks by
Priority. Then you have to change the priority of each task. There are 5 default priorities: Highest, High, Medium, Low, Lowest.
But you can also add new priorities or change existing priorities if needed.