Cal. Health & Safety Code § 7150.25

(a) Subject to Section 7150.35, a donor or other person authorized to make an anatomical gift under Section 7150.15 may amend or revoke an anatomical gift by either of the following:
1. (1) A record signed by any of the following and recorded in a donor registry database:
  1. (A) The donor.
  2. (B) The other person.
  3. (C) Subject to subdivision (b), another individual acting at the direction of the donor or of the other person, if the donor or other person is physically unable to sign.
2. (2) A later-executed document of gift that amends or revokes a previous anatomical gift or portion of an anatomical gift, either expressly or by inconsistency.
(b) A record signed pursuant to subparagraph (C) of paragraph (1) of subdivision (a) shall comply with all of the following:
1. (1) It shall be witnessed by at least two adults, at least one of whom is a disinterested witness, who have signed at the request of the donor or the other person.
2. (2) It shall state that it has been signed and witnessed as provided in paragraph (1).
(c) Subject to Section 7150.35, a donor or other person authorized to make an anatomical gift under Section 7150.15 may revoke an anatomical gift by the destruction of the document of gift or cancellation of the document of gift on a donor database registry, or the portion of the document of gift used to make the gift, with the intent to revoke the gift.
(d) A donor may amend or revoke an anatomical gift that was not made in a will by any form of communication during a terminal illness or injury addressed to at least two adults, at least one of whom is a disinterested witness. The witnesses shall memorialize this communication in a writing and sign and date the writing.
(e) A donor who makes an anatomical gift in a will may amend or revoke the gift in the manner provided for amendment or revocation of wills or as provided in subdivision (a).