How to Amend the Constitution
One Way (used MOST) Two-thirds (2/3) of both houses of Congress vote to propose an amendment THEN Three- fourths (3/4) of the state legislatures approve it
Second Way (used once) Two-thirds (2/3) of both houses of Congress vote to propose an amendment THEN Ratifying conventions in three-fourths (3/4) of the states approve it. This method has been used only once -- to ratify the 21st Amendment -- repealing Prohibition.
Third Way (never used) Two-thirds of the state legislatures ask Congress to call a national convention to propose amendments. (This method has never been used.) Three-fourths of the state legislatures approve it
Fourth Way (never used) Two-thirds (2/3) of the state legislatures ask Congress to call a national convention to propose amendments. (This method has never been used.) Ratifying conventions in three-fourths (3/4) of the states approve it.
Informally: Can be changed by: Circumstances change
Informally: Can be changed by: Judiciary