That's hard to say. I imagine it would start at the lower courts. Take Arizona for example. If they finish up the audit and prove that Trump won the state, a judge could rule to reinstate, but I don't know how it would work after that. There would be pushback for sure, so it would likely go to the Supreme Court. Nobody wants to hear that though, since we know the SC would likely reject the case like they did already. But maybe this is exactly what they were waiting for. Perhaps they will be forced to reinstate after overwhelming evidence of fraud is proven through forensic audits.
To answer your question:
Just like they did in 1994...
https://www.baltimoresun.com/news/bs-xpm-1994-02-20-1994051024-story.html
Fraud has no statute of limitations, so proving it once and for all would be enough to reinstate the rightful winner.
Who reinstates in this case?
That's hard to say. I imagine it would start at the lower courts. Take Arizona for example. If they finish up the audit and prove that Trump won the state, a judge could rule to reinstate, but I don't know how it would work after that. There would be pushback for sure, so it would likely go to the Supreme Court. Nobody wants to hear that though, since we know the SC would likely reject the case like they did already. But maybe this is exactly what they were waiting for. Perhaps they will be forced to reinstate after overwhelming evidence of fraud is proven through forensic audits.