You can select the tasks (checkboxes on the left of tasks in a project) > pencil icon > bulk edit the task status, which is the most common way of bulk editing tasks in a project.
Or if you really don't need sections of tasks, select the parent (which selects all its children too) and delete the section. Parents reflect the children, not the other way around. So you can't cancel a parent task and have the children automatically follow suit.
If this helped you, please mark correct to help others : )