What can I do if my back is a bit hunched in the middle due to long hours in office. Any exercises I should do?

Get checked. Get check up first to see if problem with spine first. Core strengthening, regular breaks with brief stretch, proper chair and positioning at the desk are all part of the treatment plan.
Stretching. Stretch every 15 minutes when you're working on the computer. You can find more stretches here: http://www.Webmd.Com/fitness-exercise/features/stretching-exercises-at-your-desk-12-simple-tips?Page=2 for good stretching and strengthening in only 15 minutes every day, check out peggy brill's book "core program: 15 minutes that can change your life.".