Neb. Rev. Stat. § 21-1974

(a) A director may resign at any time by delivering written notice to the board of directors, its presiding officer, or to the president or secretary. (b) A resignation is effective when the notice is effective unless the notice specifies a later effective date. If a resignation is made effective at a later date, the board may fill the pending vacancy before the effective date if the board provides that the successor does not take office until the effective date.