What is the usual flight-control principle used by a hot-air balloon pilot to change altitude?

A hot-air balloon pilot changes altitude primarily by changing the temperature of the air inside the envelope.

Heating the enclosed air with the burner makes it less dense than the surrounding atmosphere, increasing lift and causing the balloon to rise. Allowing the air to cool, or releasing hot air through the top valve, reduces lift and causes descent.

A balloon can usually drift with the wind at its current altitude, but it does not have an airplane-style engine and rudder for direct horizontal steering. Pilots therefore seek wind layers moving in useful directions by climbing or descending.

The basket does not steer the envelope, and inflating a wing is not part of normal hot-air balloon operation. Altitude control is the key practical form of flight control.