Trump can't cancel the elections as it's in the Constitution. It's up to congress. But let's say he does it - then there are no reps who get elected as their year ends I think Jan 3rd, 2021 so there is no House of Representatives then and then the Senators who terms expire will not be there and the POTUS's term expires so he's gone. There will be more Den Senators then and it appears that Sen Leahy will them assume the Presidency.