While it might not be possible to 'undo' severe arthritic changes in the spine, you can make your back feel much better with a new active lifestyle and a back exercise program.   My most complete reference to date on caring for your back is the eBook,Correct Posture. 

Has your back been uncomfortable recently? You can start with our  exercises for lower back pain: a routine of gentle spine movements and stretches that relieve stress and discomfort in the back. Also see my lower back pain remedy article.  It includes ergonomic recommendations for lying down, standing and sitting at your desk. There are exercises to relieve stiffness and you'll also learn the best position to relieve lower back discomfort. 

The longer term solution: build an active lifestyle from activities you enjoy and include specific  back stretching exercises and strengthening exercises into your weekly routine.