Uncomplete a Task

Create a new task group.
The task status is unique to each task. Or put another way, a single file may have many tasks associated with different statuses. So when you create a new task group that will automatically create a new task that's status is reset.

A file can be perpetually upgraded. A single task is an atomic unit of work. So if you realize you want to further work on a completed task set with dataset ABC simply create a new task group with ABC (and say the new Label Schema) and you will have fresh tasks. Use the isolate/mirror settings to control data sharing between tasks.

Going forward, to prevent a task from easily becoming complete, consider enabling Reviews. A review loop can bounce multiple times before reaching Complete.

At the moment we don't have a a way to uncomplete a specific existing task, partly becuase of the events system since downstream events may depend on a complete action. It's not impossible but it would mean handling a lot of edge cases or clarifying that uncomplete doesn't undo downstream events etc. If you have a strong use case for needing to uncomplete a task and neither of these solutions work for you please create a git issue.