My name is Tomas Duran Back until 1996,I held a C15 CSL, I let It expired. After 25 years, I wonder if I can renew/reactivate that contractor's license. How do I renew an expired license?

Actually, if it was a sole owner license, you can submit an application for original license, take the exams, and request to have the old license number reissued.

https://www.cslb.ca.gov/Contractors/Maintain_License/Renew_License/Failing_To_Renew_Your_License.aspx A CSLB license is renewable within the first 5 years of being expired. After 5 years, the license is no longer renewable. One would have to apply for a new license.