Plurality voting with single member districts leads to two party systems. It would require seriously amending the Constitution to change that.
Actually it wouldn't take amending the Constitution [which says nothing about requiring plurality or First-Past-the-Post voting], only changing Federal election laws, in order to completely break the plurality system.
First, there are two states (Maine and Nebraska) where the Electoral College vote can be split; increasing which states with this system would then magnify the value of 3rd-party efforts [as each such state greatly increases the odds of a minor candidate earning the one or two electoral vote(s) which might deadlock the EC, forcing the election to be determined by the House instead]. As seen by the fact this system already exists, this change could be implemented without requiring changes to the Constitution or federal election laws, only state laws.
Secondly, change could be instituted within the House of Representatives by revising the laws on how members are elected: Federal law requires the current separate district methodology but we could move towards a state-level proportional representation system. This would grant easier third-party access to Congress and, while not directly contributing to Presidential aspirations, would elevate the visibility of those platforms and policies. Again, this change would not require a Constitutional amendment, but only altering existing Federal election laws.
Because FPTP/plurality voting sustains the current two-party system even in the face of such hatred the electorate shows for Clinton and Trump, saying these changes do not require amending the Constitution does seem to discount the resistance these changes would face... but I believe the unprecedented hatred for those two candidates and the extreme partisanship on display by their supporters together indicate the importance of making them.