To convert an existing subtask into a task, you must first open the task that contains the subtask.
From the Subtasks section, click the "More options" icon to the right of the subtask and select Convert to task from the drop-down menu.
You will then be able to change the name of the new task and assign members to it.
NOTE
The newly created task will be added to the same project and to the same task list as the original task.