You have a couple of options:

1) You can Customize and enable the Order column on that page. Sort by that column and you'll be able to click on the arrows to move items up the list or directly to the top.

2) Use the Backlog Item Planner page (new in Release 6.4). This page allows you to drag and drop the tasks (or tests) within a backlog item. Click on the Backlog Item Planner link in the Action menu to the right of your backlog item.