Balloon rising: When the air inside the balloon is heated it expands, due to which its density decreases and the surrounding cold air has more density. 

Because the hot air is less dense, the buoyant force acting on the balloon due to the difference in density between the heated air inside and the cooler air outside becomes greater than the weight of the balloon.

As a result, when the buoyant force exceeds the gravitational force pulling down on the balloon, it rises into the air.

Balloon sinking: When the air inside the balloon while in air is cooled down the density of air inside the balloon increases. 

With denser air inside, the buoyant force acting on the balloon decreases. If this force falls below the weight of the balloon , it can no longer support it.

Consequently, when gravity overcomes buoyancy, the balloon begins to sink.