Welcome to FS. Yes - you can edit previous entries on your diet calendar - just hover your mouse on the item and you will see that it turns to a "link" - click it an on the page that appears, you can edit the amount - like if you had indicated that you'd had one and a half servings initially but really had only 1 - you can adjust the amount - look at the right side of the page.
And yes, you can save stuff a couple of ways. One FS does it for you. When you are on the Food Diary page, under each of the meals (Breakfast, Lunch, Dinner, etc.) you will see "+ Add Item" - this is what you use (obviously) to add food eaten - once you have entered something, FS retains it there (might just be the 15 or so most recent items), or you can do a "search" - so if you have already entered Milk, it will be listed there - just click it again. Then you get a window to be able to indicate how much of that item you ate.
You can also create your own "food". For instance, I used to make a salad everyday for lunch (wonderful salad bar at work). I would make the same salad every day - the first time listing each item in the lunch area. Then from that I could create My Favorite Meal - you just give it a name and then it is saved - you'll find it on the lower right side of the Food Diary page.
Forgive me if I missed some steps - it's been a while since I've done this.
About the weight issue - I've never tried it - but it looks like you can only adjust the weight for the current day. Nothing I tried allowed me to go back to a previous day.
Don't be afraid to just poke around click on things - you find out a lot that way.
Hope this helps, Joyce
"Diet" is a four-letter word.
"You don't have to see the whole staircase. Just take the first step." Martin Luther King, Jr.