Hello Mandy, I've been wondering about this myself. I don't know if it is possible, but it would very (very!) practical if it was. For example, for adding information that wasn't on the package. It just doesn't make sense that you have to create a new entry from scratch, just to modify it.
But then, I don't know if changing a certain value in a registered food item would then mean that nutrition for previous days for other people (who were using this food entry) would automatically be recalculated.