The current system for electing the president can be changed by the states without a constitutional amendment.
The National Popular Vote proposal would ensure that the candidate with the most votes in all 50 states and DC would be elected president. It is a state law, under which states agree that they will assign all of their electors to the candidate who has the most votes in all 50 states and DC. It does not go into effect until enough states have ratified it that that candidate will be sure to win. That will be when the states that have passed it collectively have at least 270 electoral votes. It has already been enacted in 11 states with 165 electoral votes.
So the system can be changed to a national first-past-the-post (usually called plurality) election without any change to the U.S. Constitution. However, that will not make it possible to elect nationwide by instant runoff voting (usually called ranked choice voting). Because that would require every state to use the ranked ballot, and to coordinate centrally to determine the winner, it would take a constitutional amendment.
To get involved with the movement for the National Popular Vote proposal, check out www.nationalpopularvote.com. To get involved with the movement for ranked choice voting, check out www.fairvote.org.