How can linear atelectasis of lungs be treated/ reversed, ?

Nothing to worry: This is almost nothing. Exercise and live will and it will go away. You probably had a lung infection and la is a residual that should go away. To be safe check with your doc.

Usually no treatment: Treatment depends on cause and severity. Minor linear atelectasis typically requires no treatment. It may resolve on its own. More severe atelectasis may be from respiratory infection, chronic lung diseases, obstruction from inhaled foreign objects, lung tumors, fluid in the lung, severe asthma and chest injuries. It does not sounds like you have those problems, however.