Click the three dots to reveal the 'More' menu.

Click on "Account information"

Click on 'Add your date of birth to your profile

Here you will be redirected to your profile page. Click 'Edit Profile'

Scroll to the birth date section. and Click "Edit"

Edit again

Manage who can view your birthday by selecting the "Who sees this" dropdown menus for the month, day, or year of your birth date. Public Your followers People you follow You follow each other Only you Choose from the following options:

When you're ready, click Save.