How do I change my birthday if I put the wrong one?

As StuartATrueRed said you can change your DOB at https://account.microsoft.com. If this was an error in the date of birth it can be changed to reflect across your account though.
You would need to provide proof of your age with a credit card in your name

Hi SlimeyBonz, just go to account.microsoft.com. Hope it's just a problem of days and not years

You can change the date of birth of your Microsoft account on account.microsoft.com. Just goto your info on the page, and change it. but it will not change your date of birth on the Xbox, as that cannot be changed.

I have already tried this but apparently the information isn’t right even thought I haven’t changed address, haven’t changed my bank details, the only thing that has changed is my email and password and I’ve lost everything because a bot sees I put the wrong birthday in because I can’t remember the birthday as I made this account before I was 18 so I put a different year in

go to account settings on your console and see if promote to adult account is there

If it isn't you must wait till the actual date of your 18 th birthday or whatever you put in originally and no it can't be changed after wards

My 18th birthday has passed. I have my ID to prove it and everything, but it's still not showing up anywhere I look. I doubt I put in the wrong date, but is there anyway to check that? If I did put the wrong date in that's extremely annoying...