How to Change Birthday?
Step 3: Scroll down and click Edit next to Birth Date or Birth Year (you’ll need to hover over the info you want to edit for the Edit option to appear)
Step 4: Use the dropdown menus to change your birthday
Step 5: Click Save Changes
You can change who can see your birthday by adjusting the audience. There are 2 audience selectors next to your birthday: one for the day and month and one for the year. Friends won’t get a notification about your upcoming birthday if you don’t share the day and month with them.
Why you can’t change your birthday?