HI Michelle,
First things first. You need to look on the food list and identify whether the food you are trying to sub is a carbohydrate, fat or protein. Then you can look on the exchange list and figure out how much you need. 4 oz of chicken you could have 4oz of ground turkey, turkey breast, cod etc. (I am looking on the lean protein list) and so on and so on with the food you want to sub. Also, as Nicole mentioned, use my fitness pal to track everything…that way when you make substitutions, you can see if the macros stayed within the guidelines of your plan. Hope that helps.
Thanks.