The Founders recognized that the Constitution that they offered the country was not perfect and would have to be altered from time to time. Just as with a contract between certain parties the Constitution had to include a means to "improve and perpetuate" the Constitution. It is found in Article V.
They provided two mean by which a change to the Constitution could be proposed. The Congress could by a two-thirds vote in each of the House and Senate propose a specific amendment. Or, two-thirds of the state legislatures could vote to convene a constitutional convention in which an amendment or amendments could be proposed, considered and voted on to forward to the states for ratification.
However the amendment(s) are presented, the ratification by three-forths of the states is required before the amendment can be added to the Constitution. The ratification by the states can be either by the state legislature or by state convention called for by the state legislature.
Although it is recognized that the Founders deliberately made the amending process difficult to achieve (so that the national charter would not be changed recklessly), nevertheless Washington warned that whenever there was a need for a modification, it should be done through the amending process and not by usurpation. In his Farewell Address he said "Let there be no change by usurpation." The improper delegation of authourity by the Congress and the usurpation of authority by the executive and judicial branches are said to be responsible for the most serious problems presently assailing the country.
(W.Cleon Skousen, The Making of America: The Substance and Meaning of the Constitution  , pg. 645
So herein is the only legal way that the Constitution can be changed, superseded or nullified.