You *used to* be able to go back in and edit later - TBH we haven't had to do that in a while, but I would try by logging in to the Disney website first (on a computer if possible) and see if you can edit them there.
If, for some reason, you can't edit in either the website or the My Disney Experience app, I would go ahead and reach out to Dining prior to your arrival if at all possible.
We have walked up to (many times) with allergies, and never had any problems, but we don't have any food allergies in our family that are unique.