Article 5 provides for formal changes to the 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.
A formal change to the Consitution involves actually changing the wording of the document itself and can only be accomplished through the process of Constitutional Amendment as set forth in Article 5, above.
The meaning of specific Constitutional phrases (as opposed to the actual wording of the document) can be changed informally by way of judicial interpretation.
When the Judicial Branch makes a decision in a court case, it explains what the words in the Constitution mean. This method of changing the Constitution informally is judicial interpretation. If the Judicial Branch makes a decision on whether the actions of the President are legal or if the laws passed by Congress are constitutional, it is judicial review. The concept of judicial review is an example of judicial interpretation. The Supreme Court established the concept of judicial review for itself in the case of Marbury v. Madison. The Constitution does not specifically give this power to the Supreme Court but the decision established that authority and it is the major check the Supreme Court has on the Executive Branch and Legislative Branch.
There are other ways of changing the meaning of the Constitution informally. It can also be changed by presidential practice or custom and usage.
Quoted portions from:
The Constitution
Correct.
The Constitution exists only in the context of its case law, its meaning determined by the Supreme Court as authorized by the doctrine of judicial review.
It was always the intent of the Framers that the Constitution be subject to interpretation per judicial review:
The generation that framed the Constitution presumed that courts would declare void legislation that was repugnant or contrary to the Constitution. They held this presumption because of colonial American practice. By the early seventeenth century, English law subjected the by-laws of corporations to the requirement that they not be repugnant to the laws of the nation. The early English settlements in Virginia and Massachusetts were originally corporations and so these settlements were bound by the principle that colonial legislation could not be repugnant to the laws of England. Under this standard, colonial lawyers appealed approximately 250 cases from colonial courts to the English Privy Council, and the Crown reviewed over 8500 colonial acts.
The Yale Law Journal Online - Why We Have Judicial Review
Indeed, the Constitution is not merely a blueprint of government, its a legal document, the culmination of centuries of Anglo-American judicial tradition dating back to the Magna Carta and before.
As Justice Kennedy noted in Lawrence:
Had those who drew and ratified the Due Process Clauses of the Fifth Amendment or the Fourteenth Amendment known the components of liberty in its manifold possibilities, they might have been more specific. They did not presume to have this insight. They knew times can blind us to certain truths and later generations can see that laws once thought necessary and proper in fact serve only to oppress. As the Constitution endures, persons in every generation can invoke its principles in their own search for greater freedom.
Indeed, the Framers did not presume to have this insight, it was the Framers intent for the Constitution to forever be a source of freedom and justice for all Americans, and for Americans to use its authority to protect their civil liberties.