U.S. President Cannot Change Marriage Equality Law
The President of the United States has a lot of power but he/she cannot change marriage equality. The U.S. Supreme Court is the one with that power.
The only current open seat on the Supreme Court was previously held by Justice Scalia, who voted against marriage equality. So replacing him with another bigot changes nothing.
It is a fairly common rhetorical flourish for presidential candidates to say something like “when elected, I will repeal” a law. But they can’t.