1) Trump's new lackey post master general screws up the system badly so mail in voting takes forever, Trump declares fraud because of the delay calls the election void.
2) Say there's too much fraud going on deploy the gestapo which he has been trial running to take over swing states to rig the vote for him.
3) Declare a national emergency due to COVID and delay the election indefinitely.
4) Actively help the Russians hack the system so the vote goes in his favor.
5) Rile up his supporters enough that they do something to the democratic nominees.
There's a really long list, the problem is even if Trump lose the elections he still has what 10 weeks where he can pull a number of shenanigans to stay in power.