I live in fesno, California and I would like to add my last name to my two older son's
Birth certificate can I do this and how would I go about doing it

Im not sure Im understanding this. So let me try to answer it based on what I think is going on.
1) either your looking to add you name to a birth certificate or

2)your looking to change the name of the child.

If your looking to put your name on then your going to have to go to court and establish yourself in court as the legal father. Request a DNA and also to be added to the borth certificate.

If it's a name change for the child. You will need to contact the mother and see how it may workout. Depending on the age of the child or children the courts may not allow for a name change. If the mother refuses to agree to a name change then you will have to go to court to request it.