What do the three branches of government do?
The People
This assumes we are talking about America. It is different in
other countries.
The three branches of government (the Executive, Legislative and
Judial) all have some form of 'say' when the constitution is being
questioned or altered and they are all part of a system of checks
and balances to make sure no branch becomes too powerful.
The Legislative branch (the House and Senate) can propose
amendments to the constitution which must be voted on and must be
passed by both bodies. Most recently, amendments have been about
pay for legislators, but some of the past big ones include women's
right to vote, prohibition of alcohol and the return of alcohol,
and limiting presidents to 2 terms in office.
The Judiciary (ie. the Supreme Court) interpret cases and laws
in terms of if they are in line with the Constitution or not (among
other things). So a case where a woman is denied the right to vote
would be in direct violation of the Constitution and restitution
would be made to her. Judicial Activism is when the judges make a
landmark decision about a more vague case that changes the way laws
are viewed across the country. Brown v. The Board of Education and
Roe v. Wade were cases which did not change the Constitution per
se, but have left lasting marks on the American legal world by
establishing a precedent against which later cases are
compared.
The Executive (ie. the President) selects the justices who serve
in the Judiciary and can veto bills from the Legislative. He can
also propose legislation, including an amendment to the
Constitution.
At the end of the day though, the first answer that the people
ultimately have the final say in challenges or changes to the
Constitution is still somewhat true, because the people elect their
representatives to the legislate and executive branches who in turn
select the justices for the Judicial branch. If the will of the
people is ignored, that politician is unlikely to be re-elected,
could be impeached and the people get to choose another
representative, ultimately, giving them the people the last
word.