Neb. Rev. Stat. § 14-101.01

Whenever any city of the primary class shall attain a population of four hundred thousand inhabitants or more as determined by the most recent federal decennial census or the most recent revised certified count by the United States Bureau of the Census, the mayor of such city shall certify such fact to the Secretary of State, who upon the filing of such certificate shall by proclamation declare such city to be a city of the metropolitan class.