We don't need a Constitutional Amendment to ensure the next president is chosen by the popular vote.
When states equalling 270 or more Electoral College votes pass the National Popular Vote Compact, all of those states agree that their Electors will vote for the winner of the popular vote. As of 2016, it has been adopted by ten states and the District of Columbia, whose 165 combined electoral votes represent 61.1% of the 270 votes needed for it to have legal force.
Sign up as a supporter of the National Popular Vote on this page. Then, write a letter to your state legislators and governor.