Cal. Evid. Code § 1402

The party producing a writing as genuine which has been altered, or appears to have been altered, after its execution, in a part material to the question in dispute, must account for the alteration or appearance thereof. He may show that the alteration was made by another, without his concurrence, or was made with the consent of the parties affected by it, or otherwise properly or innocently made, or that the alteration did not change the meaning or language of the instrument. If he does that, he may give the writing in evidence, but not otherwise.