You can detach, or unlink, a subtask from its parent task. This is available at any level of subtask and does not alter any of the task data. If the detached subtask has its own subtasks, these will still be attached.

First select the subtask you want to detach and open its task card.
Second open the task card options via the three dots on the top right.
Third select Turn into a task. A confirmation prompt will appear to make sure you are doing the action you intended.
Fourth confirm and the subtask will be detached from its parent task.

Here is a quick GIF: