Once a task has been completed you can keep it around as a completed tasks. But, after a while you'll want to clean up your tasks dashboard and remove those old completed tasks.

To delete multiple tasks it's easiest to perform this action from the Tasks dashboard.

When you land on the Tasks dashboard the default view shows incomplete tasks. To show all completed tasks click on the filter and unselect "Incomplete". Then select "Completed". This will display all completed tasks.

Now that you are viewing all completed tasks, click on the multi-select option for all tasks. This will highlight those tasks in purple. Click on the actions menu and choose Delete.