How can I change my birthday if I can’t edit it on my phone?

Update the birth date associated with your Apple ID - Apple Support - https://support.apple.com/HT204164 - "Be sure to enter your correct date of birth. After you change it to any age below 13, you can't edit it."

Hello,

Please go to the website as mentioned on the screen appleid.apple.com and then edit your profile by clicking the edit button as shown on the screen. You can change your birthdate there. Make sure your apple device works on the same apple id then only it will reflect in the device.

I hope this will help you.