How would I do this? For example when I try to change one item in a smoothie recipe for just one serving it ends up going back to the previous days and changing all the macros.
You currently have two options:
1) For a one-off change, just log the original recipe to your diary and then use the 'Explode Recipe' action to break it back into the ingredients list, and make your needed modifications right in the diary
2) Save a copy of the recipe and make changes to the new one. If you plan to never use the old version of the recipe, you can use the 'Retire' action in the recipe editor to hide it from your list of foods.
There's also a trick I learnt from another user a while back: by using negative numbers in the food log to subtract unwanted things.
Say you didn't use bananas in your smoothie today, choose banana under Add Food, then add a negative number to the serving size. That would subtract the banana from your smoothie.
If your smoothie recipe has undergone more or less a permanent change, though, I'd go with Aaron's option #2.
If you have a smoothie that varies every day here's the method I use for my dinner salad that varies every day depending on what I have on hand - sort of a more detailed version of Aaron's #2 above.
This allows me to quickly add a long list of veggies I use in my salads, varying serving sizes and which ones I use easily with out having to look up each one - and I only had to set it up once.