How do I make a task to be a sub-task and vice versa?

To make a task to be a sub-task of the prior task in the tasks list, you should swipe the task from left to right.
Swiping the task from right to left will return the task to its previous state.
Note that sub-tasking is available only if tasks order is Manual (tap the title at the top of the tasks view to change tasks order).

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