What is the best way of treating or straightening lumber spine?

Well it depends on what is going on with your lumbar spine. The lumbar spine is naturally curved. “Straightening” it may be the wrong goal for the issue that you are trying to address. The exception is those few with significant scoliosis or abnormal curvature of the spine who require surgical management.

It has to do with the actual details of the damage, but in general core strengthening exercises help, avoiding prolonged seating as well. Talk to your doc about other modalities.