How is the US Constitution amended?

Answer 1

The Constitution of the United States , as provided in Article 5, may be amended when two thirds of each house of Congress approves a proposed amendment (approval by the president is not required), and three fourths of the states thereafter ratify it, sometimes within a set period. Congress decides whether state ratification shall be by vote of the legislatures or by popularly elected conventions.

Answer 2

I believe that the the US Constitution itself provides that an amendment may be proposed either by the Congress with a two-thirds majority vote in both the House of Representatives and the Senate or by a constitutional convention called for by two-thirds of the State legislatures.