Birth certificate

Get a lawyer mamsh. You can't simply change your baby's last name just because you want to. If previously married ka and you want to change your baby's last name back to your maiden last name, get a lawyer. If you're getting married to someone else and your baby's last name is sa biological father nya, you have to get the biological father's approval, and you're still getting a lawyer kasi aampunin ng mapapangasawa mo ung baby mo. If wrong spelling ang last name, follow this link https://psa.gov.ph/civilregistration/problems-and-solutions/wrong-spelling-0