I would change the results of the 1916 election: Woodrow Wilson vs. Charles Evans Hughes. I think Hughes would have been a great President, would have had the country ready for World War I quicker than Wilson did, could have forged a far better peace than Wilson did following the war, wouldn’t have clinged to power with the help of his wife because he was an invalid for the last two years of his Presidency, and probably would have saved the country from the Harding Administration.
My Democratic bias is totally going to show, but my new four faces on Mount Rushmore would be FDR, Eisenhower (there’s one Republican!), LBJ, and Clinton.