At some point the ballots are printed. I suppose that when that happens a change isn’t possible.
There are several ways that a president or congress can postpone an election - But those ways are declarations of war, imposition of martial law and total suspension of government for budget concerns.
If any of these happen, guarantee that the US population will be in full-scale revolution.
Neither the President nor Congress can "postpone an election", especially since the Constitution has no provisions for Presidential elections.
It's in the hands of 50 State Legislatures, and I expect they will do their duty.