Well most of the time it can simply by changes in lifestyle. I managed to reduce mine simply by walking everywhere ang getting fitter and losing weight.

Otherwise it may need treatment by your Doctor.
References :

Hypertension can not be cured because blood pressure is controlled by the heart, arteries and endocrine hormones. Arteries looses its elasticity due to arteriosclerosis. It is an irreversible process. Hypertension is to be controlled by diet, exercise and drugs.
Eat a low-cholesterol, low-fat diet. This kind of diet includes cottage cheese, fat-free milk, fish (not canned in oil), vegetables, poultry, egg whites, and polyunsaturated oils and margarines (corn, safflower, canola, and soybean oils). Avoid foods with excess fat in them such as meat (especially liver and fatty meat), egg yolks, whole milk, cream, butter, shortening, lard, pastries, cakes, cookies, gravy, peanut butter, chocolate, olives, potato chips, coconut, cheese (other than cottage cheese), coconut oil, palm oil, and fried foods.
Low sodium intake.(Sodium chloride, Monosodium glutamate, Sodium bicarbonate)
*Regular exercise.
*Stop tobacco smoking.
*Stop consuming alcohol.
*Reduce fat and oils in the diet.
*Obesity - In obese subjects, losing a kilogram of mass generally reduces blood pressure by 2 mmHg. (Abdominal circumference should be less than 100 cms)
*Control Diabetes Mellitus.
*Avoid worry and stress.
*Avoid occupational, aircraft and roadway noise exposure.
Please see the web pages for more details on Hypertension.
References :
http://en.wikipedia.org/wiki/Hypertension
http://www.nlm.nih.gov/medlineplus/ency/article/000468.htm
http://www.merck.com/mmpe/sec07/ch071/ch071a.html