Well, there are 2 ways.

The Gunigundo Law (RA no. 9048) allows a person to change his 1st name by applying with the Local Civil Registrar and submitting the required documents, paying the fees and complying with the publication of notices. Note that any changes in the entry to the Certificate of Birth of the applicant is limited to the change in 1st name, or correction of clerical errors.

The other way is to go to the RTC and file a petition for change of name under Rule 103 of the Rules of Court. The rules here are more stringent and the petitioner should give sufficient proof and reasonable cause before the RTC can favorably grant the petition.