You can click on the task twice, and click on the trash icon in the main panel.
Or you can right-click on the task, and click on Delete in the context menu.
Tip: Another way is to simply select a task and press Ctrl + Delete (or ⌘ + Delete for Mac).
You can click on the More Options icon at the upper right corner of the detail panel, and select Delete this task.
To undo what you just did, you can click Undo at the bottom center of your workspace.
To see your previous operations list, or the tasks completed/deleted by other members, click on the undo icon at the upper right corner. Or just hit Ctrl + Z (⌘ + Z for Mac).
Note: The undo icon only remembers the modifications that were made within 6 days.