In X-Men: Days of Future Past, Magneto is shown commanding one of the Sentinels to
"Do what you were made for!"
Why would the Sentinels accept any voice command from Magneto?
In the scene where Magneto infiltrates metal into the bodies of each of the sentinels, there's a "blink and you'll miss it" animation where the metal invades the CPU of the robot, activating it and (presumably) allowing Magneto to take control of it.
Quite how he reprograms them isn't described in detail but as the old adage goes, there's no access like physical access.