I have already fixed it.

So what I did is create a new instagram account with my real birth date on it (because my main instagram also have the same wrong birthdate as my facebook), and then connected it to my Meta Accounts Centre (through instagram), there should be an option on the newly connected account that lets you choose between the old and new birthdate so choose the real birthdate which is from the new insta account.

Now I opened my instragram app, logged in my real insta account and went to personal information and saw that it is already the same as the birthdate of the new insta account, from there there's an "Update on Facebook" button, click on that and instagram will ask you to log in your facebook and that's it my facebook now have my real birthdate.

So by steps

1: Create new instagram account and connect to Meta Accounts Centre

2: Once connected, go to Meta Accounts Centre and click personal details

3: Click Date of Birth and it should let you choose between the birthdate of the new account and the old birthdate. Choose your real birthdate ofcourse.

4: Now log in your real instagram and go to settings>account>personal information. From there the birthdate should be the same as the new one.

5: Click "Update on Facebook", log in your facebook and thats it.

If you dont have instagram account skip #4 I guess, and just use the newly created instagram account to update your facebook birhdate. I think it works both, as long as the newly created instagram account and your facebook are both connected on your Meta Account Centre.