Feeding a diet that is inappropriate for the species can cause diarrhea as for example feeding a diet that contains too much fruit. Feeding fish to box turtles causes diarrhea. (Feeding fish to American Box Turtles is not recommended.) Parasite infection also can cause diarrhea.
If your turtle has diarrhea, it is recommended that you also have a fecal exam done by a veterinarian.
Diarrhea dehydrates, so if you have a box turtle with diarrhea, I recommend twice-daily soaks for 1/2 hour in a small container with a bit of warm water until the the turtle is well again.
For recent diarrhea (measured in days), you can try and remedy it by changing the diet. Review the diet of your turtle and make sure it is the recommended diet for the species you have. Adjust the diet and see whether there is improvement. Make sure the turtle is well hydrated.
If changing the diet does not resolve the diarrhea within a day or two, the animal must be examined by a veterinarian. Exams can fecal exams.