If you’ve just recently changed your date of birth, you may have to wait a few days before you can change it again. To help make sure that people are using their real identities on Facebook, there’s a limit to how many times you can change your birthdate. Once you’re able to change your birthday again, follow the steps here: fb.com/help/563229410363824.

But, when you hit the limit your profile will be locked to change your birthdate. To change your locked birthdate you can try to ask Facebook staff to do it for you.
First log into your Facebook account (from your favourite web browser). Click on this link (fb.com/help/contact/233841356784195) and select the day month and year of your birthday. Then type a short reason why you want to change your birthday. And click submit. Just in a few seconds Facebook will send a notification to your email and tells you that the Facebook team had changed it for you.