Let's hope the elections are free and fair, and give a clear, uncontested, result.
If that does not happen, the only option is to take it to the court. At that point the court may side with Biden, or it may side with Trump. The courts opinion will be definition be the legal ruling on the election, and so decide the legal winner. There is just no other option.
In the spirit of compromise, anyone who is interested in a functional democracy must abide by it, just as with Bush v. Gore. This is why it is important to have a full court, to avoid the chaos a split court would result in, and why a contested result must be resolved by the court.