I took a look at one of your Events and I see that there is a drop down arrow for the year 2014 for the Your birthdate question. By selecting that drop down you can change the year for the birthdate.
Is this person trying to change the form after they've registered? I don't believe they can get back in to fill out the form again if they've completed the registration. Once I select a date, I am able to edit it if I haven't saved the registration.
This sounds like this may be an issue on their end. If you find others are reporting this as well, please let us know. We will need to know the Internet Browser that they are using as well.
However, I have found this registrant's details within your Event and I will show you how to edit this information for your reporting.
This will update your records. I hope that helps and let us know if you run into any trouble!