Click on the settings page, top right of FoodStorm. When you choose an order number in FoodStorm to begin with you should look to see which invoice number you are up to in your accounting package. When you transfer your orders across to your accounts the order number becomes the invoice number. To insure no data is overridden and your orders get transferred make sure you start with a unique order number.
If you process other orders in your accounts package but not in FoodStorm you might choose to make the numbers go up by 2.
The setting is available on all plans and all editions.