Read this support article about how to update the birth date associated with an Apple ID: Update the birth date associated with your Apple ID - Apple Support
How you can change it depends upon various factors described in the article.
If you still have questions or problems:
To contact Apple, click here --> Choose your country or region - Official Apple Support
Select your country (also look for "other" regions), 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 by telephone --> Contact Apple for support and service - Apple Support