Batch update the birthdays in my iPhone contacts
If you want to just clear the birthday field programmatically, you could do that on a Mac that syncs contacts with the iPhone (either via iCloud or iTunes).

Open up AppleScript editor, paste in the following and hit run to clear every birthday in your address book.

tell application "Address Book"
    set thePeople to every person
    repeat with thePerson in thePeople
        set the birth date of thePerson to missing value
    end repeat
    save
end tell


If you have the birthday data already in some other format and are looking to import it, that could be done, but would be a bit trickier. You'd probably need to get the AppleScript to look up the birthday for a given person from a file. If you need something like that, please edit your question to be more specific and give some sample data if possible.