Corp. Code, § 5224

(a) Unless otherwise provided in the articles or bylaws and except for a vacancy created by the removal of a director, vacancies on the board may be filled by approval of the board (Section 5032) or, if the number of directors then in office is less than a quorum, by (1) the unanimous written consent of the directors then in office, (2) the affirmative vote of a majority of the directors then in office at a meeting held pursuant to notice or waivers of notice complying with Section 5211, or (3) a sole remaining director. Unless the articles or a bylaw approved by the members (Section 5034) provide that the board may fill vacancies occurring in the board by reason of the removal of directors, or unless the corporation has no members pursuant to Section 5310, such vacancies may be filled only by approval of the members (Section 5034).

(b) The members may elect a director at any time to fill any vacancy not filled by the directors.

(c) Any director may resign effective upon giving written notice to the chairperson of the board, the president, the secretary or the board of directors of the corporation, unless the notice specifies a later time for the effectiveness of such resignation. If the resignation is effective at a future time, a successor may be elected to take office when the resignation becomes effective.