The Constitution may be adapted through an amendment. Amendments, as delineated in Article V of the Constitution, can be proposed by 2/3 of the Congress or by a 2/3 majority of a convention called by Congress at the request of 2/3 of state legislatures. To be ratified, as 27 amendments have been, the amendment has to be approved by 3/4 of state legislatures or 3/4 of conventions called in states.
The Supreme Court can also decide to interpret the Constitution in a different way. For example, in the case McCullough v. Maryland (1819), the court ruled that the Congress had the power under the Necessary and Proper Clause of the Constitution (Article I, Section 8) to charter the Second Bank of the United States.
In addition, the President, through custom and practice, can decide to execute some parts of the Constitution and choose not to execute others (though he or she can't change the Constitution). For example, the Constitution calls for the President to provide the Congress with information about the state of the union from "time to time." Presidents such as Thomas Jefferson delivered written updates to Congress; however, since the Wilson administration, the President has addressed the address to Congress in person. The clause in the Constitution provides the President with some degree of latitude about how he or she chooses to address the Congress, but the custom is now to provide a speech to Congress.