What would happen if a Presidential Candidate died or was assasinated between the convention and the election?

I am a political activist in the UK and a close follower of American politics. This afternoon someone asked me the above question, and I genuinely did not know the answer.

Would the VP candidate automatically take over ? Or would there have to be an emergency convention to nominate a replacement ? What if it happened a few days before the election? Would the main opposition candidate automatically win or would the election be postponed so another candidate could be selected?

I would appreciate serious cross referenced answers please. Thank you.