if the motor is AC, u can reverse rotation by swapping 2 of the 3 mains lines that you hook up to the motor
if the motor is DC, you can reverse rotation by using either a shunt or separately -excited motor (NOT a series motor)
in this case, you just flip the polarity of the input if it's sep-excited
you flip the motor terminals (but not the field circuit) if it's shunt
for a miniature door, it sounds like a low-power kind of deal, just use DC...it's easier, more widely available, and easy to flip polarities by just using some kind of relay scheme to the input.