The formal methods for changing states constitutions are legislative proposals, ballot initiatives, referendums, and .

The answer is Constitutional conventions.

Explanation:

A Constitutional convention refers to a meeting in which the main purpose is writing a new constitution or review some aspects regarding an existing constitution. The members who belong to a Constitutional convention, usually called "delegates", are often elected by popular vote.  

Some Constitutional conventions have been made in different countries to establish or to review a nation's Constitution. They include the United States, through the Philadelphia Convention in 1787, the National Convention of 1792 that took place in France, and the Quebec Conference in 1864, as well as the London Conference of 1866 in Canada.