Click the Servings button on the Planner to reveal the serving number for each of your planned recipes.
Change the serving amount of any recipe and click the Update link that will appear to the right to save the changes. The number of servings for that meal and the associated amount of ingredients in your shopping list will be adjusted for the new quantity.
To turn the Servings boxes back off press the Servings button again and it will turn from blue back to grey.
Please note: Editing the serving amount in your planner does not change the original recipe, it only changes the serving amount for that instance of the recipe on your planner.