Exercise bikes can burn off belly fat and stomach fat as long as you also improve your diet by cutting out high-calorie foods and ride the stationary bike often enough and with enough speed to where you're actually sweating.
Just slowly pedaling the exercise bike won't burn off enough calories to reduce the belly fat or stomach fat so you really have to pedal hard and break out in a sweat on the stationary bike for it to be effective.
Besides riding the stationary bike to burn belly fat you should also add 2 tablespoons of apple cider vinegar to your diet and drink it either in a glass of water or take it directly off the spoon.
The apple cider vinegar will reduce your body fat percentage and help burn off belly fat and reduce your weight.