Article V of the United States Constitution.
The Congress, whenever two thirds of both houses shall deem it necessary, shall propose amendments to this Constitution, or, on the application of the legislatures of two thirds of the several states, shall call a convention for proposing amendments, which, in either case, shall be valid to all intents and purposes, as part of this Constitution, when ratified by the legislatures of three fourths of the several states, or by conventions in three fourths thereof, as the one or the other mode of ratification may be proposed by the Congress; provided that no amendment which may be made prior to the year one thousand eight hundred and eight shall in any manner affect the first and fourth clauses in the ninth section of the first article; and that no state, without its consent, shall be deprived of its equal suffrage in the Senate.
– Article V, U.S. Constitution
According to Article V, there are two methods to propose amendments to the Constitution.
1. Congress can propose amendments to the Constitution at any time if 2/3 of both houses of Congress agree.
2. A Convention of States can propose amendments if 2/3 of states submit applications for such a convention. These applications must all deal with the same issue (i.e., limiting the power and jurisdiction of the federal government).