Amending the Constitution
The Constitution allows for amendments (changes to the constitutional contract) to be proposed by either the Congress or the states.
Constitution Article V
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;
Breaking down Article V, there is a two-step process to amend the Constitution; proposal and ratification:
Step 1) Propose Amendments
By Congress: 2/3 of House and 2/3 of Senate propose amendments to Constitution.
By State Legislatures: 2/3 of the States Legislatures (34 states) call for a convention through Congress to propose amendments.
Step 2) Ratify Amendments
¾ of the State Legislatures (38 States) ratify the amendment.
¾  of the state convention delegations ratify the amendment