You can modify the nutritional data in the nutritional information editor.
You can reach the editor either by clicking on the question mark next to the ingredient in the Item Selector window or by clicking on the magnifying glass next to the ingredient in the Nutrition tab of a recipe.
This will open the nutritional information window. Click then on the Edit button to display the editor.
To edit the nutritional data, you can either input the new nutritional information manually, or import it from the USDA national nutrient database for standard reference by clicking on the import (left-most) button of the toolbar.