:The US is clearly a representative democracy, ability to change the constitution has nothing to do with it.  On top of that, as you point out, it is possible for the US government to amend its constitution ("amend" meaning, in effect, "change").  It's just that it requires a super majority of 66% in both houses of Congress, and then it has to be ratified by 3/4 of the state governments, so it is pretty rare for an amendment to be successful.