Under Article V, amending the U.S. Constitution is a Two Step Process
Proposal – Congress or States may Propose an Amendment
Congress can propose an Amendment with a 2/3 majority vote (67 Senate / 290 House), OR when 2/3 of State Legislatures (34) call for a Constitutional Convention – which has never been used
Ratification – Only States may Ratify an Amendment
3/4 of the States (38) must ratify an Amendment through State Legislatures or Special State Conventions that are proposed by Congress or at a Constitutional Convention