Article 176. of the Family Code as amended by Republic Act No. 9255: 

Illegitimate children shall use the surname and shall be under the parental authority of their mother, and shall be entitled to support in conformity with this Code. However, illegitimate children may use the surname of their father if their filiation has been expressly recognized by the father through the record of birth appearing in the civil register, or when an admission in a public document or private handwritten instrument is made by the father. Provided, the father has the right to institute an action before the regular courts to prove non-filiation during his lifetime. The legitime of each illegitimate child shall consist of one-half of the legitime of a legitimate child.

Dahil naman kasal na si Juan at Juana at anak naman Juan si ang iyong anak. Lalo at gusto na ng ama na ipagamit ang kanyang Surname sa kanyang anak. Applicable sainyo ang Art. 176 ng family code.

Ang process lang ay kailangan mag file ng “Affidavit to use the Surname of the Father” (AUSF) sa Office of the Civil Registrar of the city or municipality where the child was born. Magpunta muna sila Juan at Juana sa Civil Registrar kung saan pinanganak ni Juana ang anak nila ni Juan. Mag tanong duon kung meron na template ng affidavit to use the surname of the father na pwede mapag gayahan at itanong din kung ilang copies. After magawa ang AUSF, ipasa ito sa Civil Registrar.

Disclaimer: Please be advised that the following information is provided solely for general reference and informational purposes. It is not intended as a comprehensive or articulate legal advice due to the absence of complete knowledge regarding the specific circumstances of the case or issue at hand. Kindly consider this as a general overview to gain a preliminary understanding of your situation