Once a turnaround has been created from a manual schedule, its date and time cannot be changed.
If you need to correct it, you'll need to delete and recreate it with the right date and time. See How can I delete a flight from a schedule?
How can I change the date and time of a turnaround created from a manual schedule?
You cannot change the date or time of a turnaround once it has been created from a manual schedule.