There are several ways you can correct the time spent on a task. If the Time Tracker is running you can correct time by simply setting the cursor at a time section and correcting it manually. If you’ve already stopped the tracker, you can correct the time when saving your progress.
Note that in the Card Details section you can only set estimated time (that option exists to compare estimated time and actual time spent). To correct the time spent you should go to the Timing section or Comments section of the right panel.