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Navigate to the folder, project, or space where the task is located and switch to List view.
Click the subtask and drag it over to the task that's supposed to become its new parent task. The new parent task is highlighted light blue.
Release your mouse.
Your subtask now lives in the task you moved it to and no longer exists in the original parent task.
Navigate to the folder, project, or space where the task is located and switch to Table view.
Locate the subtask in the table, click the dots icon to the left of the subtask name and drag it over to the task that's supposed to become its new parent task.
Release your mouse.
You can do the same on the table portion of Gantt Chart.