How do I edit and manage task boards?

To edit and manage the task boards in a project, open the popup dialog for a sprint and use the "Edit this board" and "Manage task boards" links next to the "Task board" field:

You can also edit the task board shown in the "Task board" page by clicking "Edit this board" at the bottom of the page:

Note that when you edit a task board, this will affect all sprints that use that board. Create a new task board instead, if you don't want to affect other sprints.