Can I decrease nearsightedness?

Not without surgery. There unfortunately is no way via diet, exercise, or exercises for the eye to reduce a refractive error. Laser vision correction though would reduce nearsightedness. To improve your vision at distance, i recommend a complete eye exam and then a discussion with your doctor of the options including glasses, contacts, or laser vision correction.
Laser surgery/glasse. There is much you can do on your own to decrease near-sightedness. This only means you can't see things far away too well, but things close up are good...So for most people, having near- is better than far-sightedness etc.. Consult an eye doc--ophthalmologist--for possible glasses, contacts, or even laser surgery if deemed appropriate for your eyes. Good luck..