I assume you're familiar with Update the birth date associated with your Apple ID - Apple Support
Did you check you junk mail? Are you using an alternative email account for notifications? About your Apple ID email addresses - Apple Support
If you still have problems:
Click here --> https://support.apple.com/choose-country-region Select your country, then a product. If you don't see one that handles your issue then keep experimenting with selections until you reach one that gets you a chat session or a telephone call and get the representative to redirect you.
or:
Contact Apple for support and service --> "See a list of Apple phone numbers around the world."
Click here --> https://support.apple.com/HT201232
Edit: I'll add that the link at the end of the article about updating birthdays is very general but if you want to use it, just do what I advise in the first contact method I mentioned above.