Answer
Vertical motion is controlled by adjusting air temperature inside the balloon.
Work Step by Step
The density of the air changes with its temperature.
Hot air is less dense than cool air. As a result, hot air rises, cool air descends.
The density of the balloon is affected by the density of the air inside it, that is, by the temperature of the air inside it.
If the balloon is surrounded by air denser than itself, the balloon moves up,
if they have equal density, then they remain in the same relative position,
and if the surrounding air is less dense, the more dense balloon descends.
