You simply swipe the task left to reveal the button(s) to delete, like most of popular apps like Messages, Mail, etc.

For recurring tasks, you have two types of delete buttons; one to delete only that recurrence (button with green check, meaning having finished the task), and one to totally delete the recurring task (cancel the whole recurrence, button with red trash can). On overdue recurrences, you will see only the finishing button with the green check. If you would like to totally delete the recurring task, you need to do that on the next (still due) recurrence of the task.

For one-time deadlined or casual tasks, finishing and deleting are one and the same thing on one button (with a red trash can and a small check sign on it together).

If the deadlined task is an Urgent/None-Left Shopping task, you can update it (by tapping on the task) for re-scheduling but you can not delete it. It is automatically deleted when you check off (meaning you have purchased) the last urgent/none-left shopping item. (Just like it is created when you add an urgent/none-left shopping item). If you do not want Urgent/None-Left Shopping tasks at all, you will find a switch for disabling them in Settings.

You can choose one of 5 categories when adding or editing a task. Of those categories, "Deadlined" and "Recurring" tasks will be under "Scheduled" tab, "As Soon As Possible", "Later On" and "Someday/Maybe" tasks will be under "Casual" tab.