What happens in the US presidential election if 1or both of the candidates die before the election?or the winner dies before taking office?

A rather large mess.

It will all be decided in the electoral college when they vote after we all do, so they will be able to vote for the appropriate candidate.    
Now if that president dies between the electoral college vote and the inauguration, then the VP gets the job. 

It's left up to the Electoral College members to choose somebody.  

If an incumbent President dies then the VP takes over.  So if Trump wins reelection and then dies, Pence would become the new President, just as he would at any other point in Trump's term.  If a challenger died after being elected then their VP nominee, who had just been elected to, would be sworn in.  Perhaps they would get sworn in as VP first and then as President.  So if Biden wins and then dies before January 20th (say from catching corona) Kamala Harris will  become the next President.  It's more speculative what would happen if a Presidential candidate died before the election.  if Trump died then Pence would become the new President.  If Biden died then presumably Democrats would elevate Harris to the Presidential slot.  We've never had a situation like that before though.

Most likely a Harris v. Pence situation. 

The answer depends in part on state law, in part on party rules, and in part on the Constitution.

Both parties have a mechanism in place for replacing a presidential or vice-presidential candidate before the election.  However, all states have a deadline for replacing a candidate on the ballot.  At this point, in almost all states, we are past that deadline.

So the current candidates will remain on the ballot.  But the names on the ballot are mere placeholders for the party's slate of electors.  And if the presidential or vice-presidential candidates die before the general election, votes for that ticket still count to elect the connected slate of electors.

Of course, after the general election is when things get legally interesting.  Some states have no laws binding electors to vote for their party's ticket.  (The electors generally will because of the way that they are chosen but they are legally "free agents.")  In the other states, the laws binding electors are written in different ways.  Some bind the electors to the candidates on the ticket; others bind the electors to their party's nominees.  Additional, some states enforce these laws by fining electors; other states enforce these laws by allowing the removal of electors who do not comply with their pledges.  Finally, some states merely direct that the electors pledge support for the candidates with no enforcement provision.  The difference in the laws could become crucial.  If the winning candidate (or both candidates) die before the electors meet, you could have a situation in which a significant number of electors vote for their party's replacement nominee but some vote for the deceased candidate or some other party leader.   This split could result in no candidate getting 270 electoral votes and the House getting to choose between the top three candidates.

If the president-elect dies after the electors meet, the Constitution is clear that the vice-president elect will become President on January 20.

The VP's step in.

Pelosi will be president. She will have to hire a good accountant as her net worth triples.

jesus christ Read the damn constitution

If the winner dies after being formally elected by the electoral college but before taking office, then whomever was elected Vice President becomes President (and appoints someone else as Vice President, subject to Senate confirmation.)

If one or both candidates die, then the election still happens.  If the winner is alive, then it does not matter that the other died.  It is unclear what happens if a dead person wins or if the winner dies after Election Day but before the electoral college meets.  One theory holds that the electoral college would pick someone; another holds that whomever was elected Vice President becomes President.