On a mobile device:
Click on the profile icon at the bottom of the screen and then the “Edit” link under your name — this will take you to the “Editing” menu, where you can change your date of birth and other personal details.
Change your date of birth and then click “Save.”
On a computer:
Click on your profile picture at the top of the screen > a drop-down menu will appear where you can find the “Settings” > select “Personal information” and change your date of birth.