5 year old kid wants to use father's last name

Hi, everyone!

I sought to ask legal advise on how to proceed on my niece's case. She's a 5-year old, and she's currently using my sister's last name (her mother). They hadn't had the chance (or just didn't do so) to change her last name to his father's. My sister went to the Civil registry and asked how to proceed and she was told to get a copy of my niece's baptismal certificate showing his father's last name, but that isn't the case, her birth certificate also carries her mother's last name. Her baptismal certificate can't be changed till her Birth certificate is changed. My sister and the father are not married but they are living together


We don't know how to proceed from here. We just want to get this processed as his Kuya is using their father's last name.

Any help would be greatly appreciated.

Thanks

the father needs to execute an affidavit of admission of paternity and submit that to the Local Civil Registry