If you've added tasks that you decide you would prefer to group under another task as subtasks, or if you'd like to take a subtask and bring it up to the top level, you can change them quickly by either drag and drop, or using a keyboard shortcut.
Drag and drop
To convert a task to a subtask, just expand the main task by clicking the 'Subtask' icon and drag the other task under it:
To change a subtask to a main task, just grab the 'handle' icon to the left of the task details and drag it above the main task:
You can drag it above the main task to bring it up to the top level, and if you'd like to bring it back down to subtask level, just make sure you've expanded the task you want to move it under, and you can drag it there:
Note - if you grab and drag a task that has subtasks under it, those subtasks will be moved with it.
If you hover the mouse over a task and press the 'Tab' key on your keyboard, that will change a task to a subtask by indenting it - or shifting it down one level:
You can also hold 'Shift' at the same time as pressing 'Tab' to bring it back up a level, too.