Many people began their Instagram account by linking to their Facebook account. In that case, your birthday will auto migrate from Facebook to Instagram. If it is incorrect, you can change it on Facebook, and it will automatically update on Instagram. Alternatively, you can change it in Instagram directly:
1. Go to your profile.
2. Tap Edit Profile.
3. Tap Personal Information.
4. Tap Birthday and enter your date of birth.
Instagram states that it doesn’t share your birthday with other people.