Clinton has never been accused of election fraud. Had she won, the validity of the election would not be in doubt. However if there is evidence she conspired to fix the general election, it should be exposed and negate her win.
Now, that said, if she’s guilty of a crime, she should immediately be impeached as someone who legitimately won an election but is constitutionally unfit to hold office. We have well understood mechanisms for dealing with impeached, legitimately-elected presidents.