Can I make changes to what I've eaten in the past?
You can edit any of the entries made in your diary at any time past or future. It's recommended however, that you update your diary on a daily basis to make it easy to remember exactly what you've eaten.
Forgetting to log any foods or drinks really does defeat the object; you're diary has to be a true representation of what you eat.
Your diary history should reflect all your habits, good and bad; only then can you make positive future changes towards your goals.