Let user fix serving size

I live in Europe and when I scan UPCs, I find some user-submitted entries that have errors in the serving size. When I click on “report error,” the app invites me to fix all nutritional info but not the serving size!! This is frustrating and keeps me from improving your database. 
 

example: today I scanned a local sour cream. The user had entered it in with a serving size of 250ml (the entire container) and had entered the nutritional info as listed. The problem is that the nutritional info is per 100g, not per ml. As a result, the calculations are all off. I’m not going to eat the whole pot, and I prefer to weigh food than use volumes, so it is really hard to use. I would be happy to correct the basic serving size /nutritional basis but the system doesn’t let me, and also doesn’t let me explain the problem to you in any easy way.

I also do not see a way to provide a new entry for the scanned item, if there already is an entry for that  barcode.