Reopen completed tasks
If a completed task is shown in your list, you can reopen it by double-clicking on it to open the Outlook task, and setting the status of the Outlook task to "Not Started" or "In progress," or by right-clicking on it, and selecting "Reopen completed task."
This will reopen the completed task and its ancestor tasks to the root level, and show them in your list. It will not reopen child tasks of the reopened task.
If the task is no longer shown on your list, you can see a list of all your completed tasks by right-clicking on your list and selecting "Show completed tasks." This will refresh the list to show all your tasks, active and completed.
Search for and select the task you want to reopen, and click Reopen completed task.