Mike Wall, writing for Space.com:
Astronauts residing on the orbiting lab receive a digital version of their ballot, which is beamed up by Mission Control at the agency's Johnson Space Center (JSC) in Houston. Filled-out ballots find their way back down to Earth along the same path.
"They send it back to Mission Control," said NASA spokesman Jay Bolden of JSC. "It's a secure ballot that is then sent directly to the voting authorities."