How Do I Put My Weight Loss Into The Tracker?

I can only change my start and end date, how do I enter the changes in between?

Btw why don't they have the BMI adjusted to people with different frames? I assume the one here is for average build but I have a very thin or fine (what would you call that?) bone structure (I still fit a bracelet that fitted me at age 7, says enough I guess ). My BMI is now very low, but for my build it would be ok and I can still loose 3 kilo and look normal.