Where do birthday balloons go when they are released in the air?

With helium filled balloons, they rise upwards as the gas inside is of a lower density than the atmosphere around them. The balloon will continue to rise and the gas inside it will expand as the atmospheric pressure decreases with increasing altitude. Eventually, the gas inside the balloon will expand enough to burst the balloon and the balloon will fall back to the ground. When and where this happens depends on weather conditions, air temperature, the rubber used in the balloon, etc.