Neb. Rev. Stat. § 21-290

(MBCA 8.07) (a) A director may resign at any time by delivering a written resignation to the board of directors or its chairperson or to the secretary of the corporation. (b) A resignation is effective when the resignation is delivered unless the resignation specifies a later effective date or an effective date determined upon the happening of an event or events. A resignation that is conditioned upon failing to receive a specified vote for election as a director may provide that it is irrevocable.