Certain exercises may prevent or delay the progression of hyper-kyphosis or hunched back, but that cannot completely correct it; it normally progresses with age.
See: Recent studies:
A journal Osteoporosis International study involved 50-59 year old women wherein "progression of kyphosis was prevented in those who did exercises to strengthen the extension muscles of the spine, three times a week for a year."
An Archives of Physical Medicine and Rehabilitation study showed that in "women ages 65 to 80 who already had noticeable kyphosis did exercises in groups for 12 weeks and then on their own for a year. Those who were followed up maintained a small improvement in their usual and best measures of kyphosis."
Stronger evidence, however, is needed before a general recommendation is made.
Summary: "The normal forward curve of the spine is also called kyphosis. Excessive kyphosis is sometimes a result of musculoskeletal disease and sometimes simply the wages of not following parental advice to stand up straight."
C. Clairborne Ray. "Stand Up Straight!" NY Times May 2, 2011