Cal. PROB § 4153

(a) The authority of an attorney-in-fact under a power of attorney may be revoked as follows: (1) In accordance with the terms of the power of attorney. (2) Where the principal informs the attorney-in-fact orally or in writing that the attorney-in-fact’s authority is revoked or when and under what circumstances it is revoked. This paragraph is not subject to limitation in the power of attorney. (3) Where the principal’s legal representative, with approval of the court as provided in Section 4206, informs the attorney-in-fact in writing that the attorney-in-fact’s authority is revoked or when and under what circumstances it is revoked. This paragraph is not subject to limitation in the power of attorney. (b) An attorney-in-fact or third person who does not have notice of the revocation is protected from liability as provided in Chapter 5 (commencing with Section 4300).