User can prioritize the stories from unassigned user story tab by dragging them up and down.
The action menu supports three options to move user stories.
Move to top
To move up a user story go to the action menu and click on move to top. Click on the user story to place it on the top of the list.
Move to bottom
To move down a user story go to the action menu and click on move to bottom. Click on the user story to place it in the bottom of the list.
Move to selected position
To move a user story to your desired position go to the action menu of the story you want to move and click on move to selected position.On clicking a pop opens for the user to select the position.Click on save. When the user clicks on save the story it will be re-positioned.