California Code, Probate Code - PROB § 6226

(a) A California statutory will may be revoked and may be amended by codicil in the same manner as other wills.

(b) Any additions to or deletions from the California statutory will on the face of the California statutory will form, other than in accordance with the instructions, shall be given effect only where clear and convincing evidence shows that they would effectuate the clear intent of the testator. In the absence of such a showing, the court either may determine that the addition or deletion is ineffective and shall be disregarded, or may determine that all or a portion of the California statutory will is invalid, whichever is more likely to be consistent with the intent of the testator.

(c) Notwithstanding Section 6110, a document executed on a California statutory will form is valid as a will if all of the following requirements are shown to be satisfied by clear and convincing evidence: