Formal method of amending the Constitution
An amendment can be proposed by two-thirds of both houses of Congress or
By two-thirds of state legislatures requesting Congress to call a national convention to propose amendments.
An amendment can be ratified by a favorable vote in three-fourths of all state legislatures or by such a vote in specially called ratifying conventions called in three-fourths of the states.
PS : This is for the US constitution, don't know if it differs for other countries.
