Seems it is disturbingly easy to do.

How to Hack an Election in 7 Minutes - POLITICO Magazine

No sooner did a team of bewildered deliverymen roll the 250-pound device into a conference room near Appel’s cramped, third-floor office than the professor set to work. He summoned a graduate student named Alex Halderman, who could pick the machine’s lock in seven seconds. Clutching a screwdriver, he deftly wedged out the four ROM chips—they weren’t soldered into the circuit board, as sense might dictate—making it simple to replace them with one of his own: A version of modified firmware that could throw off the machine’s results, subtly altering the tally of votes, never to betray a hint to the voter. The attack was concluded in minutes.