Using firm but gentle force, AAA says to push on the door, then pull if the car door is frozen shut. It might take a few tries, but keep the motions gentle and firm so as not to break your door ...

Use lubricants like WD-40 or petroleum jelly swiped over a door lock to prevent freezing. Blow compressed air into door locks with the nozzle to force out moisture. Invest in a good car cover.