A constitution is a vital part of any country's system of government. It provdes the rules by which the people agree to be governed. It is a contract, the terms and conditions under which those elected must govern, be they a monarch, president or parliament.
The people as a collective are the ultimate supremacy. Once accomplished, a constitution cannot be changed without the express will of the people for to do so would be a violation of contract and would render the authority of those in breech null and void.