Signals Notebook: Can we change the status of a "Completed" Task ?

A completed task can be reopened only if you have it in the Tasks To do Information Item as shown in screenshot below:

We cannot delete/re-open a completed Task from the Task Container tab. 

Only the assigned Analyst of a task can reopen a completed task. Therefore, the Analyst can navigate to the experiment containing the Tasks To Do table, identify the relevant task, and reopen it as needed.

You can find detailed explanation in User Guide -  Signals Notebook > Tasks and Requests > Creating a Request and Managing Tasks.