The time to have done this was on November 4th. As soon as the election became suspect (which was the evening of the 3rd) Trump should have turned out the Marshal’s Service to impound the machines in suspect States/Counties. Too much time has passed now to show any malfeasance — especially since the chain of custody (and connectivity) is now under doubt.
It’s one of the two most exasperating things about Trump. He hesitated too easily to take action on many issues that affected his term. The second was his abysmal selection of talent — which likely affected the first.