Before editing a workout, check your diary to be sure that “not synch” does not appear on the workout.

If you edit before the workout has been synced, you might create a duplicate with no workout data.

Then, to edit the workout:

You can delete a workout by going to Diary and pressing the workout you want to delete for 2 seconds. The option to delete it will pop up.

Please note that deleting a workout from your phone will delete it from the web service as well.