To add more time to a completed task, you can either:
Edit the remaining time by opening the task and going to the log tab:

Or click the checkbox in list view and choose "Leave logged time," then choose how much time to reschedule:

To add more time to a completed task, you can either:
Edit the remaining time by opening the task and going to the log tab:
Or click the checkbox in list view and choose "Leave logged time," then choose how much time to reschedule: