My ex husband has been giving me a hard time about my kids for the past almost 2 1/2 yr. I am getting remarried and my fiance wants to become their legal stepfather, how can we do that legally?
You don't need to do anything to have your new husband be your kids' stepfather, that's what he'll be when you get married. He's not adopting them, and they have a dad, so there's no reason to change their names. Besides, your kids may want to keep their dad's name, and it sounds like it would only make things worse with your ex.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.
I agree with Ms. Koslyn but if you do decide to attempt to change their names you will need to petition the Court. The right forms are available in the pull down menu at http://www.courtinfo.ca.gov/forms/. You can be sure your ex will oppose it.