what happens if someone has changed my birthday in my Apple ID

First, are you sure it was from Apple? Scams (e-mail, text messages, and phone calls) are getting very good at closely imitating true Apple communications. Always be cautious. These support articles have some guidelines:

About identifying legitimate emails from the App Store and iTunes Store --> Identify legitimate emails from the App Store or iTunes Store - Apple Support

Recognize and avoid phishing messages, phony support calls, and other scams - Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support

Avoid scams when you use Apple Cash --> Avoid scams when you use Apple Cash - Apple Support

Apple 'How to identify, avoid, and report phishing' video--> https://youtu.be/SR3Z3fXXjfw

About Gift Card Scams --> About Gift Card Scams - Official Apple Support

If you are uncertain about a message and a resource provided in that message, do not click on any links in the message. Try to independently verify the resource by going to a support article page on apple.com and use the instructions in the article to verify though Apple itself, or your Apple device features such as Settings or an Apple app, what the message is claiming. To ask Apple start at this web page: Official Apple Support

Sign in to https://appleid.apple.com at any time to manage your account and to verify your information there is correct.

Visit: https://appleid.apple.com

Tap on Personal Information at the left, and view your birthday.

If that date is correct, then the message lied.

There are too many scams and too many fraudulent advertisements around, unfortunately.

Many are trying to get you to expose sensitive info (your Apple ID and password, here) or install some problematic apps.

Oh, and if you encounter the words “VIRUS” or “HACKER” online, assume it’s an advertisement or entertainment.

tbirdgoldcoast wrote:

I think someone has used my name for a new Apple ID account because the Apple ID that they used is not what I use for my Apple ID. In this case what do I do about that?

Is your birthday correct on your Apple ID?

If so, you have identified the message as phishing, no doppelgänger Apple ID exists, and there is no additional work necessary.

If your birthday is not correct, then you have a far larger problem; an Apple ID compromise.

PS: Sending email addresses and calling telephone numbers and sending numbers for text messages can all be faked.

tbirdgoldcoast wrote:

The notification email that I received was identical to Apple that its frightening that the scammers are so good.

Yes; they are good.

So too are the folks creating political propaganda, advertisements, “news” and entertainment programming, and other fodder.

Social media platforms are marvelously insidious, too.

And transgressions are blamed on “the algorithm“ should some shenanigans get noticed, of course.

If you want to see someone exceedingly good at this stuff, see some of the YouTube videos by Rachel Tobec.