A proportion of Donald Trump's supporters believe that the presidential election that has recently led to Joe Biden being sworn in was fraudulent. As I recall, numerous states, such as Arizona, are currently under pressure to audit their elections. Suppose that one such audit happens and finds very strong evidence of fraud that happened in favor of Joe Biden. What happens then? Has this ever happened before? If not, what do the laws say?
Note: The key part of this question is how the finding of fraud would affect a president that has already entered office.