California Code, Probate Code - PROB § 15640

(a) As provided in the trust instrument.

(b) In the case of a revocable trust, with the consent of the person holding the power to revoke the trust.

(c) In the case of a trust that is not revocable, with the consent of all adult beneficiaries who are receiving or are entitled to receive income under the trust or to receive a distribution of principal if the trust were terminated at the time consent is sought. If a beneficiary has a conservator, the conservator may consent to the trustee's resignation on behalf of the conservatee without obtaining court approval. Without limiting the power of the beneficiary to consent to the trustee's resignation, if the beneficiary has designated an attorney in fact who has the power under the power of attorney to consent to the trustee's resignation, the attorney in fact may consent to the resignation.

(d) Pursuant to a court order obtained on petition by the trustee under Section 17200. The court shall accept the trustee's resignation and may make any orders necessary for the preservation of the trust property, including the appointment of a receiver or a temporary trustee.